Lil: A Scripting Language

Lil is part of the technology that powers Decker, a multimedia creative tool inspired by HyperCard. Decker uses Lil for adding custom behavior to decks and the widgets within. Lil is designed to be learned in layers, but it is a richly multi-paradigm language which incorporates ideas from imperative, functional, declarative, and vector-oriented languages.

on mode a do   # line comment
	each x in a
	extract first key orderby value desc from r


Lil’s implementation needs to be fairly small, as an interpreter is shipped with every standalone Decker document along with the rest of the runtime. At the same time, Lil should be prepared for the everyday needs of Decker users, with primitives to handle common use-cases and a minimum of boilerplate.

The language therefore tries to thread the needle between a design which is simple and a design which is ergonomically pleasant. There is a small number of datatypes, features and primitives, which compose in many useful ways. The query syntax generalizes to manipulation of dictionaries, lists, and strings, and replaces many individual primitive operations in other languages with a single algorithmic framework. Simple things are easy, and complex things are possible.

Types and Conversions

There are 7 types of value in Lil: numbers, strings, lists, dictionaries, tables, functions, and interfaces.

Lil has a soft, spongy, dynamic type system in which values do their best to convert to a more relevant type as the need arises.

When indexing into values, we will uniformly refer to elements:

The number zero and the empty string, list, or dictionary are all falsey, and any other value (including any table, function, or interface) is truthy.

Lil, the Imperative Language

Lil scripts are a sequence of expressions, including literal values, references or assignments to variables, conditionals (if), each and while loops, and function declarations (on).

Variable and function names may contain any alphanumeric characters (as well as ? and _), but must not start with a number. Variable assignments use the symbol :, read as “gets” or “becomes”:

a: 23
longer_name: "a string"

If a value has never been explicitly placed in a variable, it contains the number 0. You can access or assign to elements of a list or dictionary by subscripting with brackets:


For convenience, indexing may also be performed with “dot notation”, where a name after the dot is treated as a string index. These expressions are equivalent:


If you index a list with integers within range of its count it will act like a list. If you index a list like a dictionary- with keys that are not integers within range of its count- it will become a dictionary.

b: 5              # b contains the number 5
b[0]: 5           # b becomes a length-1 list containing the number 5
c                 # c contains the number 0
c.fruit: "yes"    # c now contains a dictionary with the key "fruit" and the value "yes"

When the leftmost part of an assignment expression is an expression- not just a variable name- the value of that expression is amended and a new value with appropriate changes is returned:

(11,22,33)[1]:44  # produces the list (11,44,33)
"Cat"[1]:"ive"    # produces the string "Civet"
().baz:99         # amend the empty list at index "baz" (promoting it to a dictionary) with the value 99,
                  # producing the single-element dictionary {"baz":99}
(11,22).baz:33    # extend (11,22) into a dictionary, and bind "baz", giving {0:11,1:22,"baz":33}

Note the important distinctions between these similar-looking cases:


foo[1]:44     # get the value in "foo", amend it at index 1, and store the amended list in "foo" again.

(foo)[1]:44   # the parenthesized portion is a subexpression which gets the value in "foo".
              # amend it at index 1, and return the amended list, leaving "foo" unchanged.

(foo[1]):44   # this is not syntactically valid!
              # assignment to a subexpression must have a dotted or bracketed index.

If you index with a dot followed immediately by another dot or open-bracket it means “each element”. Use this for drilling into nested lists or dictionaries:


t[1]        # "CD"             # the 1st element of t
t.[1]       # ("B","D","F")    # the 1st element of each element of t


d           # {"a":{"key":"apple"},"b":{"key":"pear"}}
d.a         # {"key":"apple"}
d.a.key     # "apple"
d..key      # {"a":"apple","b":"pear"}

Lil has a number of unary and binary operators- see appendix 1 and 2 for details. Many, like + and *, should be familiar from mathematics or other programming langugages. The most important thing to remember in Lil is that expressions have uniform operator precedence: expressions are always carried out right-to-left, unless explicitly parenthesized:

3*2+5    # 21
3*(2+5)  # 21
(3*2)+5  # 11

You can make decisions with if. The keyword if is followed by a conditional expression, one or more statements, and finally the keyword end. The statements inside the if ... end will only be executed if the conditional expression has a truthy value. You can also optionally include an else keyword to divide the body of the if into a truthy half and a falsey half.

if 1>2
	"narp"  # this doesn't happen

if 5
	"yarp"  # 5 is truthy, so this does happen!

if 5
	"also yarp"  # this happens...
	"also narp"  # but this does not!

You can iterate over the elements of a value with an each loop. The keyword each is followed by zero or more variable names, the keyword in, an expression giving the source to iterate over, one or more statements forming the “body” of the loop, and finally the keyword end. The source is always treated as a dictionary, and for each iteration of the loop the variable names provided are assigned to the value, key, and index of the current pairing in that dictionary. For a list, key and index will naturally be identical. Here are a few examples of each loops:

each x in 3,5,7

each val key in d

each x in range 10

You can also repeat a body of code an indefinite number of times with a while loop. The keyword while is followed by a conditional expression, one or more statements forming the “body” of the loop, and finally the keyword end. The conditional expression is evaluated before each iteration of the loop, and the loop stops if this ever results in a falsey value. Here’s an example:

while a>3
	print[a]     # prints 5, then 4, then stops.

To declare a function, use the keyword on followed by a name, zero or more argument names, the keyword do, one or more statements comprising the “body” of the function, and finally the keyword end. To call a function, use its name and a set of bracketed expressions corresponding to the arguments it takes. Extra arguments are ignored, and missing arguments are bound as 0:

on pair x y do

pair[3 5]    # returns a list of 3 and 5
pair[3]      # returns a list of 3 and 0
pair[3 5 7]  # returns a list of 3 and 5

pair[3,5]    # careful: ',' joins items into lists- it is not an argument separator!
pair 3 5     # wrong: this is three separate statements, not a function call!

Lil, the Functional Language

As we’ve seen above, thinking about Lil as an everyday garden-variety imperative language is perfectly sufficient for writing scripts. Some characteristics, though, make it well-suited to the functional style of programming, in which we aim to minimize mutation, compose our program from functions which do not have “side-effects”, and make use of so-called “higher-order” functions- functions which take functions as arguments.

Values in Lil are truly values. They have copy-on-write semantics: an assignment to part of a value creates an entirely new value, leaving any other references to the original value unchanged:

show[a]  # 1,2,3
show[b]  # 1,5,3

Statements in Lil are always expressions. That is, they always return a value and can be composed within larger expressions. Assignments evaluate to the value being assigned. if returns the value of the last statement in its taken half. (A missing else just means the falsey half evaluates to 0.) while likewise evaluates to the last statement on the last iteration of its body:

x:y:z                       # assign both x and y the value of z
a: if x>5 99 else 33 end    # assign a to either 99 or 33
c: while b<100 b:b*2 end    # assign c to 128

The each loop collects together the results of each iteration of the loop and returns them. If the input was a dictionary, the output will be a dictionary with the same keys. Otherwise, the output will be a list. Some other languages call this operation map:

each x in 3,5,7   # (300,500,700)

each v in t       # {"foo":3,"bar":5}
	count v

You can pass functions to other functions by name:

on apply func do
on twice x do
apply[twice]    # ("two","two")

Since on...end is an expression, you can also directly substitute it into the call, just as with “anonymous” or “lambda” functions in other languages.

apply[on thrice x do x,x,x end]

One advantage of functions always having a bound name is that there’s no special combinator plumbing required for anonymous recursion. If the name never matters, consider using the name _ to signal your intent.

Lil uses lexical scope: variables will resolve to the closest nested binding available, and the local variables of a caller to a function will not be visible or modified by the callee (unless the callee’s definition is nested in the caller):

on quux x do
	print[local]           # 99
on zami x do
	print[global,local,x]  # 333,23,77

Furthermore, functions close over variables in their lexical scope, allowing for encapsulated “objects” with their own mutable state:

on counter x do
	on inc do
print[a[]]        # 101
print[a[]]        # 102
print[b[]]        # 201
print[a[]]        # 103
print[x  ]        # 0

Whenever an assignment is carried out, if the variable name in question has been assigned to in any surrounding lexical scope, the assignment will update the closest definition. If the name has never been assigned to before, a new local variable will be created at the current scope. The on, each, and query (select, update, extract) statements always create new local variables for their arguments, loop variables, or columns, and can thus shadow (take precedence over) outer local variables of the same name. When in doubt, use unique names; it’s much less confusing!

Finally, if you’re mentally wed to the idea of expressing algorithms recursively, you may be pleased to discover that Lil supports tail-call elimination. If a recursive function calls itself (or another function) as the final operation in the function, it will not consume extra stack space:

on addrec x y do
	if x>0
		1+addrec[x-1 y]  # addition happens after call returns. not tail-recursive!

on addtail x y do
	if x>0
		addtail[x-1 y+1]  # tail-recursive call.
addtail[80000 5]  # this won't blow the stack.

Lil, the Query Language

So far, we haven’t looked much at tables. While one can operate on tables by indexing them or iterate over their contents with an each loop, the real power of tables comes from Lil’s query syntax, which resembles a simplified version of SQL. Unlike some of their SQL counterparts, select, extract, update, and insert are all pure operations which strictly return a new value as a result.

select is used for querying a table, and obtaining a table of results. It is followed by a sequence of column expressions (possibly with names), optionally a where, by, and/or orderby clause, each of which should produce a list of results for each row, the keyword from, and then finally an expression giving the source table. In the context of these clauses- and when computing result columns- the columns of the table will be bound as variables.

In a select, extract or update, clauses are carried out in the following order:

Given a simple table:"Alice","Sam","Thomas","Sara","Walter"
people:table people

Let’s perform some selections:

select from people       # select all columns, like "select *" in SQL
# +----------+-----+--------------+
# | name     | age | job          |
# +----------+-----+--------------+
# | "Alice"  | 25  | "Developer"  |
# | "Sam"    | 28  | "Sales"      |
# | "Thomas" | 40  | "Developer"  |
# | "Sara"   | 34  | "Developer"  |
# | "Walter" | 43  | "Accounting" |
# +----------+-----+--------------+

select name from people      # select a specific column
# +----------+
# | name     |
# +----------+
# | "Alice"  |
# | "Sam"    |
# | "Thomas" |
# | "Sara"   |
# | "Walter" |
# +----------+

select firstName:name dogYears:7*age from people     # compute and rename columns
# +-----------+----------+
# | firstName | dogYears |
# +-----------+----------+
# | "Alice"   | 175      |
# | "Sam"     | 196      |
# | "Thomas"  | 280      |
# | "Sara"    | 238      |
# | "Walter"  | 301      |
# +-----------+----------+

select name where ("S%m" parse name) from people     # where takes a boolean (0/1) column
# +--------+
# | name   |
# +--------+
# | "Sam"  |
# | "Sara" |
# +--------+

select name index orderby name asc from people     # orderby takes a column of values and "asc" or "desc"
# +----------+-------+
# | name     | index |
# +----------+-------+
# | "Alice"  | 0     |
# | "Sam"    | 1     |
# | "Sara"   | 3     |
# | "Thomas" | 2     |
# | "Walter" | 4     |
# +----------+-------+

select name job by job orderby name asc from people     # sort names within each group
# +----------+--------------+
# | name     | job          |
# +----------+--------------+
# | "Alice"  | "Developer"  |
# | "Sara"   | "Developer"  |
# | "Thomas" | "Developer"  |
# | "Sam"    | "Sales"      |
# | "Walter" | "Accounting" |
# +----------+--------------+

select name job orderby (job join name) asc from people     # sort by multiple columns
# +----------+--------------+
# | name     | job          |
# +----------+--------------+
# | "Walter" | "Accounting" |
# | "Alice"  | "Developer"  |
# | "Sara"   | "Developer"  |
# | "Thomas" | "Developer"  |
# | "Sam"    | "Sales"      |
# +----------+--------------+

All result columns will be repeated to match the count of the longest result. Thus, if all the new columns yield a non-listy result, the group is effectively collapsed into a summary row:

select employed:(count name) job by job from people
# +----------+--------------+
# | employed | job          |
# +----------+--------------+
# | 3        | "Developer"  |
# | 3        | "Developer"  |
# | 3        | "Developer"  |
# | 1        | "Sales"      |
# | 1        | "Accounting" |
# +----------+--------------+

select employed:(count name) job:(first job) by job from people
# +----------+--------------+
# | employed | job          |
# +----------+--------------+
# | 3        | "Developer"  |
# | 1        | "Sales"      |
# | 1        | "Accounting" |
# +----------+--------------+

When computing columns, you’re working with lists of elements, and taking advantage of the fact that primitives like < and + automatically “spread” to lists. If you want to call your own functions- say, to average within a grouped column- write them to accept a list like so:

on avg x do ((sum x) / count x) end

select job:(first job) avg_age:avg[age] by job from people
# +--------------+---------+
# | job          | avg_age |
# +--------------+---------+
# | "Developer"  | 33      |
# | "Sales"      | 28      |
# | "Accounting" | 43      |
# +--------------+---------+

The same applies to any functions called in a where, by, or orderby expression. The predefined aggregation functions sum, raze, min, and max may come in handy!

The update statement has the same syntax and clauses as select, but behaves differently: results are merged with the original table, and the original order of rows is preserved. orderby can be used with update, but will only impact the inputs to column expressions, not the order of the result rows.

update job:"Engineer" where job="Developer" from people
# +----------+-----+--------------+
# | name     | age | job          |
# +----------+-----+--------------+
# | "Alice"  | 35  | "Engineer"   |
# | "Sam"    | 28  | "Sales"      |
# | "Thomas" | 50  | "Engineer"   |
# | "Sara"   | 44  | "Engineer"   |
# | "Walter" | 43  | "Accounting" |
# +----------+-----+--------------+

As with select you can compute new named columns- values will be filled in with 0 for rows masked off by a where clause.

update manager:random[name] where job="Developer" from people
# +----------+-----+--------------+---------+
# | name     | age | job          | manager |
# +----------+-----+--------------+---------+
# | "Alice"  | 25  | "Developer"  | "Sara"  |
# | "Sam"    | 28  | "Sales"      | 0       |
# | "Thomas" | 40  | "Developer"  | "Sara"  |
# | "Sara"   | 34  | "Developer"  | "Sara"  |
# | "Walter" | 43  | "Accounting" | 0       |
# +----------+-----+--------------+---------+

When mixing queries with other types of code, it may be very useful to execute a query and get back simple lists, strings, or numbers. The extract statement is another variation on select which unpacks its result directly into a value, instead of a result column in a table:

jobs:extract first job by job from people
# ("Developer","Sales","Accounting")

extract gives you access to a variety of useful operations for tables, dictionaries, or even lists:

extract value orderby value asc from jobs              # sort a list
# ("Accounting","Developer","Sales")

extract index orderby value asc from jobs              # grade a list
# (2,0,1)

extract value orderby index desc from jobs             # reverse a list
# ("Accounting","Sales","Developer")

extract list index by value from "ABBAAC"              # group a list
# ((0,3,4),(1,2),(5))

extract list value by floor index/3 from "ABCDEFGHI"   # partition a list
# (("A","B","C"),("D","E","F"),("G","H","I"))

extract first value by value from "ABBAAC"             # distinct items in a list
# ("A","B","C")

If names are specified, all results are collected into a dictionary:

extract a:first age b:last age orderby age asc from people 
# {"a":25,"b":43}

The insert statement adds new rows to a table. It is followed by a sequence of one or more column names followed by a colon and an expression, the into keyword, and an expression evaluating to a table.

insert name:"John" job:"Writer" age:32 into people
# +----------+-----+--------------+
# | name     | age | job          |
# +----------+-----+--------------+
# | "Alice"  | 25  | "Developer"  |
# | "Sam"    | 28  | "Sales"      |
# | "Thomas" | 40  | "Developer"  |
# | "Sara"   | 34  | "Developer"  |
# | "Walter" | 43  | "Accounting" |
# | "John"   | 32  | "Writer"     |
# +----------+-----+--------------+

Missing columns will be filled in as 0. If any of the inserted values are lists, the remaining columns will be spread to match:

insert name:("John","Eric") age:32 zodiac:("Taurus","Virgo") into people
# +----------+-----+--------------+----------+
# | name     | age | job          | zodiac   |
# +----------+-----+--------------+----------+
# | "Alice"  | 25  | "Developer"  | 0        |
# | "Sam"    | 28  | "Sales"      | 0        |
# | "Thomas" | 40  | "Developer"  | 0        |
# | "Sara"   | 34  | "Developer"  | 0        |
# | "Walter" | 43  | "Accounting" | 0        |
# | "John"   | 32  | 0            | "Taurus" |
# | "Eric"   | 32  | 0            | "Virgo"  |
# +----------+-----+--------------+----------+

As a special case, if the value to insert into is a number, treat it as an empty table. This is a convenient way to make a table from scratch:

insert name:("John","Eric") age:32 zodiac:("Taurus","Virgo") into 0
# +--------+-----+----------+
# | name   | age | zodiac   |
# +--------+-----+----------+
# | "John" | 32  | "Taurus" |
# | "Eric" | 32  | "Virgo"  |
# +--------+-----+----------+

Lil also offers two basic joining operations: join (natural join), and cross (cartesian/cross join):

jobs:insert job:"Sales","Developer","Accounting","Facilities" salary:85000,75000,60000,50000 into 0
# +--------------+--------+
# | job          | salary |
# +--------------+--------+
# | "Sales"      | 85000  |
# | "Developer"  | 75000  |
# | "Accounting" | 60000  |
# | "Facilities" | 50000  |
# +--------------+--------+

people join jobs
# +----------+-----+--------------+--------+
# | name     | age | job          | salary |
# +----------+-----+--------------+--------+
# | "Alice"  | 25  | "Developer"  | 75000  |
# | "Sam"    | 28  | "Sales"      | 85000  |
# | "Thomas" | 40  | "Developer"  | 75000  |
# | "Sara"   | 34  | "Developer"  | 75000  |
# | "Walter" | 43  | "Accounting" | 60000  |
# +----------+-----+--------------+--------+

guests:insert name:"Alice","Joan","Oscar","Thomas" into 0
# +----------+
# | name     |
# +----------+
# | "Alice"  |
# | "Joan"   |
# | "Oscar"  |
# | "Thomas" |
# +----------+

select a:name b:name_ where name < name_ from guests cross guests
# +---------+----------+
# | a       | b        |
# +---------+----------+
# | "Alice" | "Joan"   |
# | "Alice" | "Oscar"  |
# | "Joan"  | "Oscar"  |
# | "Alice" | "Thomas" |
# | "Joan"  | "Thomas" |
# | "Oscar" | "Thomas" |
# +---------+----------+

For a row-wise join- concatenating tables with the same columns- you can simply use ,, which has the same padding/widening behavior as insert. You can likewise take or drop rows or columns from a table. If the left argument to take is a list of numbers, it picks out those rows very much like a generalization of select:

"name" take people
# +----------+
# | name     |
# +----------+
# | "Alice"  |
# | "Sam"    |
# | "Thomas" |
# | "Sara"   |
# | "Walter" |
# +----------+

("age","job") take people
# +-----+--------------+
# | age | job          |
# +-----+--------------+
# | 25  | "Developer"  |
# | 28  | "Sales"      |
# | 40  | "Developer"  |
# | 34  | "Developer"  |
# | 43  | "Accounting" |
# +-----+--------------+

3 drop people
# +----------+-----+--------------+
# | name     | age | job          |
# +----------+-----+--------------+
# | "Sara"   | 34  | "Developer"  |
# | "Walter" | 43  | "Accounting" |
# +----------+-----+--------------+

(0,2,3) take people
# +----------+-----+-------------+
# | name     | age | job         |
# +----------+-----+-------------+
# | "Alice"  | 25  | "Developer" |
# | "Thomas" | 40  | "Developer" |
# | "Sara"   | 34  | "Developer" |
# +----------+-----+-------------+

The flip of a table transposes its data, promoting the first column (or, if it exists, a column named key) from the original table to column keys for the result:

expenses: insert kind:"tax","gas","power","food" jan:11,22,33,44 feb:55,66,77,88 into 0
# +---------+-----+-----+
# | kind    | jan | feb |
# +---------+-----+-----+
# | "tax"   | 11  | 55  |
# | "gas"   | 22  | 66  |
# | "power" | 33  | 77  |
# | "food"  | 44  | 88  |
# +---------+-----+-----+

flip expenses
# +-------+-----+-----+-------+------+
# | key   | tax | gas | power | food |
# +-------+-----+-----+-------+------+
# | "jan" | 11  | 22  | 33    | 44   |
# | "feb" | 55  | 66  | 77    | 88   |
# +-------+-----+-----+-------+------+

The key column makes the flip of a table a reversible operation. If you don’t need the original keys, you can discard them with drop:

"key" drop flip expenses
# +-----+-----+-------+------+
# | tax | gas | power | food |
# +-----+-----+-------+------+
# | 11  | 22  | 33    | 44   |
# | 55  | 66  | 77    | 88   |
# +-----+-----+-------+------+

In some situations you may wish to construct or query tables containing columns which have names that are reserved keywords (like count or range) or are not valid Lil identifiers (like a name with spaces). To define such a column, enclose the name in double-quotes:

denormal: select "with \"escapes":index "count":value from "ABC"
# +---------------+-------+
# | with "escapes | count |
# +---------------+-------+
# | 0             | "A"   |
# | 1             | "B"   |
# | 2             | "C"   |
# +---------------+-------+

insert "pet name":"Galena","Pippi","Chester" "pet species":"Chicken","Chicken","Toad" into 0
# +-----------+-------------+
# | pet name  | pet species |
# +-----------+-------------+
# | "Galena"  | "Chicken"   |
# | "Pippi"   | "Chicken"   |
# | "Chester" | "Toad"      |
# +-----------+-------------+

To reference these columns in a query, every column expression has the variable column bound to the entire table (or subtable, when grouping) within column expressions. As usual, you can access specific columns by indexing this table with a string:

select where column["with \"escapes"]>0 from denormal
# +---------------+-------+
# | with "escapes | count |
# +---------------+-------+
# | 1             | "B"   |
# | 2             | "C"   |
# +---------------+-------+

extract column from denormal
# +---------------+-------+-------+--------+-------+
# | with "escapes | count | index | gindex | group |
# +---------------+-------+-------+--------+-------+
# | 0             | "A"   | 0     | 0      | 0     |
# | 1             | "B"   | 1     | 1      | 0     |
# | 2             | "C"   | 2     | 2      | 0     |
# +---------------+-------+-------+--------+-------+

Extracting column may occasionally be useful for debugging complex queries!

In general, use select for narrowing down or summarizing tables, update for making changes to a table while preserving or extending its existing structure, extract for pulling data out of a table for use elsewhere, and insert to make new tables or append to existing ones.

It may also be helpful to think in terms of how the shape of output tables relates to input tables for various table operations:

Query Statement Output Rows Output Columns
select <= input any number
update = input >= input
insert >= input = input
extract <= input n/a
join <= x * y < x + y
cross = x * y = x + y
flip x columns x rows

Lil, the Formatting Language

The parse and format primitives are used for breaking strings apart into Lil values, and formatting Lil values into a customizable string representation, respectively. Both take as their left argument a format string with a concise notation for controlling these processes. In many cases, these format strings are symmetrical: the parse of the format under the same Format string will be an identity operation, and vice versa:

f: "0x%04h"       # a four-digit zero-padded lowercase hexadecimal number with an '0x' prefix
f format 123      # "0x007b"
f parse "0x007b"  # 123

Format strings consist of a sequence of patterns and literals. Patterns always begin with a % character, may contain several optional flags, and end in an alphabetic character. Pattern flags configure the details of how each pattern behaves, and may appear in the structure %*-0N.DX, where N and D may be 1 or more digits 0–9, and X is a pattern type. All flags are optional. If present, their meanings are as follows:

Pattern types are as follows:

Type Null Parsed Formatted
% n/a literal % character. * is implied. %
n n/a number of chars that have been read. nothing.
m n/a matched? value is 1 iff the format has matched so far. nothing.
z n/a value is 1 iff the format matches and read the whole input. nothing.
s "" string. read N chars or until next literal. any string, up to D chars.
u "" uppercase string. just like s, but converts to uppercase. any string, up to D chars, converted to uppercase.
l "" lowercase string. just like s, but converts to lowercase. any string, up to D chars, converted to lowercase.
r "" repeat. 0 or more (or N) characters within a valid set. any string, exactly N chars if specified.
o "" optional. 0 or 1 (or N) characters within a valid set. any string, exactly N chars if specified.
a () ASCII. reads like s; value is list of ASCII ordinals. list of ASCII ordinals converted to a string.
b 0 reads like s; value is 1 iff first char is in tTyYx1. any value to true or false based on truthiness.
f 0.0 Lil float. allows any number of decimals. show D decimal places or however many are needed.
c 0.0 currency. parses values like -$1.23 to Lil floats. show D or 2 decimal places, like -$1.23.
i 0 signed integer. signed integer.
h 0 hexadecimal integer. parses lower- or uppercase. format int as hexadecimal in lowercase.
H 0 hexadecimal integer. parses lower- or uppercase. format int as hexadecimal in uppercase.
j 0 a JSON value. any value to a JSON string.
e 0 read ISO-8601 date-time into a unix epoch int. format unix epoch int as ISO-8601 date-time.
p () read ISO-8601 date-time as a dictionary of time parts. format dict as ISO-8601.

When parsing, each pattern will be matched against input in sequence, consuming some number of input characters and producing output values, and literals will be expected; if at any point a pattern or literal fails to match against input, parsing will cease, and any subsequent patterns in the format string will yield appropriate “null” values. Thus, a given format string will always yield a fixed number of results, no matter the input. In the case where there is exactly one value-yielding pattern in the format string, parse output will simply be that value instead of a list.

"%f %s %i" parse "12 apples"                        # (12,"apples",0)
"%f %ss" parse "12 apples"                          # (12,"apple")
("amount","noun") dict "%f %ss" parse "12 apples"   # {"amount":12,"noun":"apple"}
"[%s]" parse "[something]"                          # "something"

The right argument to parse may be list of strings; in this case the output will be a list of rows, where each row contains the values from parsing one input string. The flip of this would give columns instead. Here’s an example of parsing a table of fixed-width records:

form: "%6s%6c%2i"
data: "apple  $1.00 1\ncherry $0.3515\nbanana $0.75 2"

r: form parse "\n" split data
#(("apple ",1,1),("cherry",0.35,15),("banana",0.75,2))

table r
#| c0       | c1   | c2 |
#| "apple " | 1    | 1  |
#| "cherry" | 0.35 | 15 |
#| "banana" | 0.75 | 2  |

flip r
#(("apple ","cherry","banana"),(1,0.35,0.75),(1,15,2))

t: table ("name","price","amt") dict flip r
#| name     | price | amt |
#| "apple " | 1     | 1   |
#| "cherry" | 0.35  | 15  |
#| "banana" | 0.75  | 2   |

("\n",form) format t              # round-trip
#"apple  $1.00 1\ncherry $0.3515\nbanana $0.75 2"

When formatting, literals will be included in the output string and each pattern will control conversion of one value from the input list, with a few exceptions as explained below. Missing arguments will be interpreted as appropriate “null” values.

"%i,%a,%i" format 1,(list 65,66,67)  # "1,ABC,0"

The left argument to format may be a list of strings; in this case it is a series of alternating delimiters and format strings, with the last item always being a format string. Each format string is “pushed” one layer deeper into the right argument of format, and (if present), the delimiter will be used to fuse these intermediate strings, allowing a simple statement to recursively format a complex structure:

                    ()  format 11,22                   # (11,22)                # (identity)
                "%03i"  format 11,22                   # "011"                  # format the first item and discard unused(!)
          (list "%03i") format 11,22                   # ("011","022")          # format each
           (":","%03i") format 11,22                   # "011:022"              # fuse (format each)
    ("<%s>",":","%03i") format (list 11,22),(list 33)  # ("<011:022>","<033>")  # format each (fuse (format each))
("@","<%s>",":","%03i") format (list 11,22),(list 33)  # "<011:022>@<033>"      # fuse (format each (fuse (format each)))

Recursive formats will “explode” tables into lists of row-lists:

t: insert alpha:"one","two" beta:11,22 into 0
#| alpha | beta |
#| "one" | 11   |
#| "two" | 22   |

(list "%u - %i") format t
#("ONE - 11","TWO - 22")

If N is specified, the %s,%a and %b patterns will read up to N characters of input when parsing. Otherwise, the next character in the format string will be interpreted as a delimiter, and input characters will be read until that delimiter is encountered. (Note that if %s/%a/%b is immediately followed by a pattern, the delimiter will be %!) If %s appears at the end of the format string, it will simply read the remainder of the input string.

The %p pattern operates on a dictionary with numeric fields for year, month, day, hour, minute, second:

"%p" parse  "2021-02-03T04:05:58Z"  # {"year":2021,"month":2,"day":3,"hour":4,"minute":5,"second":58}
"%p" format ().year:1984            # "1984-01-00T00:00:00Z"

The %n pattern can be used for progressive parsing. It also offers a way of finding the first index of a given character in a string:

data: "one,two,three"
"%s,%n" parse data         # ("one",4)
"%s,%n" parse 4 drop data  # ("two",4)

"%*sA%n" parse "BBCABA"    # the index of the first 'A' is 4

The %m and %z patterns can be used to disambiguate between failing to match and successfully parsing a null-equivalent value. You can also do some kinds of pattern matching:

"%i%m" parse "23"                      # (23,1) # successful parse
"%i%m" parse "0"                       # (0,1)  # successful parse of 0
"%i%m" parse "orange"                  # (0,0)  # mismatch, defaulted to 0

"exe%m" parse "foo.exe","execute"      # (0,1)  # prefix match
"exe%z" parse "execute","exe"          # (0,1)  # full match

The %r pattern is followed by one or more “valid” characters, the count given by D (or 1 by default). This pattern matches and collects input if and only if the input characters are within this set of valid characters. The - flag inverts this behavior, such that the pattern matches and collects only characters which are not in the valid set. If N is specified, exactly N characters must be matched; otherwise %r will accept zero or more valid characters. The %o pattern is exactly like %r, but if N is unspecified it will accept zero or one valid characters:

"%.2r01"     parse "01110201"     # "01110"          # grab any leading number of binary digits
"%*.2r01%z"  parse "010","012"    # (1,0)            # string consists only of binary digits?
"%r-%i"      parse "----45"       # ("----",45)      # grab any prefix of minus signs
"%o-"        parse "A","-A","--A" # ("","-","-")     # grab at most one minus sign
"%*o-%i"     parse "-45"          # 45               # discard any sign, and read an unsigned int
"#%-r\n\n%s" parse "# comment\nA" # (" comment","A") # read a Lil-style line comment until a newline

The %j pattern can be used to format or parse data as JSON. When formatting JSON, Lil dictionary keys will be cast to strings, and anything other than a number, string, list, or dictionary will become a JSON null:

"%j" format (11,22) dict (33,44)    # "{\"11\":33,\"22\":44}"
"%j" format list 11,22              # "[11,22]"
"%j" format table 11,22             # "null"

When parsing JSON, the value true will become the number 1, and false or null will become the number 0. This JSON parser is highly tolerant and will among other things accept non-string JSON values as dictionary keys, missing , and : delimiters, and some missing trailing delimiters. Postel’s Law, baby!

"%j" parse "[true,false,null,1]"   # (1,0,0,1)
"%j" parse "{11:22,33:44"          # {11:22,33:44}

Lil, the Vector Language

Lil has a number of features influenced by “Vector-oriented” languages like APL, J, K, and Q. The most obvious is conforming, in which a number of primitive operators like + and - can be applied either to single numbers or entire lists. This functionality is essential to how Lil manipulates columns within queries.

Let’s start with a unary operator. Applied to a list, - “penetrates” to each list element:

-(5)                # -5
-(10,-35)           # (-10,35)

With a binary operator, a non-list item will “spread” and be paired with each element in a list. Given two lists, corresponding element from each list are “paired up”:

100+10              # 110
100+(10,20)         # (110,120)
(100,200)+10        # (110,210)
(100,200)+(10,20)   # (110,220)

The same pattern is carried out recursively; you can operate on arbitrarily high-dimensional data this way. Sometimes you may still need an each or flip to get your operands to line up the way you want:

2*(list 1,2,3),(list 3,4)
# ((2,4,6),(6,8))

2 cross 3
# ((0,0),(1,0),(0,1),(1,1),(0,2),(1,2))

each x in 2 cross 3 do (27,19)+x end
# ((27,19),(28,19),(27,20),(28,20),(27,21),(28,21))

flip(27,19)+flip 2 cross 3
# ((27,19),(28,19),(27,20),(28,20),(27,21),(28,21))

Conforming is why Lil has two different equality operators: = (equals) conforms, and ~ (match) does not. Consider these cases:

22=11,22,33          # (0,1,0)
22~11,22,33          # 0

(11,22,33)=11,22,33  # (1,1,1)
(11,22,33)~11,22,33  # 1

()=11,22             # ()
()~11,22             # 0

In many situations, = and ~ are equivalent. Prefer ~ when you don’t need conforming behavior; it signals your intent more clearly to a reader, since it is easy to tell without context that the result will be a single number.

Combining conforming operators with reducing operators like sum and raze offers many elegant and direct solutions to problems. Compare each of these approaches to counting how many times a value needle can be found in a list haystack:

needle:   "apple"
haystack: ("frog","apple","chicken","toadstool","apple","rice","fish")

c:0 each x in haystack if x~needle c:c+1 end end c     # imperative

c:0 each x in haystack c:c+x~needle end c              # imperative, without a conditional

extract count value where value=needle from haystack   # query

count needle take haystack                             # functional (filter)

sum needle=haystack                                    # vector-oriented (spread-conforming =)

Lil, the Decker Language

Lil contains a number of language features and builtins specifically intended for interaction with the Decker environment.

The send statement is followed by the name of a function, and then a set of arguments in brackets, just as if the function were being called. Instead of calling the named function directly, send finds the next closest binding (or the most recently shadowed binding, if you like) for the name and calls that function. This is particularly useful for event handlers which need to “bubble” events up to a more general handler, like a card’s link definition deferring to the deck’s link definition.

The send statement can also be handy to override built-in functions like go[] with your own code, so that you have a chance to perform some work before, after, or instead of their default behavior. Defining these overrides in the deck’s script will make them apply for events triggered from any card or widget:

on go x do
	# provide our own novel 'special name':
	if x~"Random"

	# use 'send' to call the original definition:
	send go[x]

An interface appears similar to a dictionary, but indexing or assignment through an interface may have side-effects, and the values in fields may change over time:

  "%e" format
  "%e" format


Interfaces cannot be defined from Lil programs- they are furnished by a host application like Decker. Consult Decker’s manual for a description of the interfaces you can use in your scripts. It is also not possible to enumerate the keys of an interface; they may have infinitely many keys, populated on the fly. Thus, the in operator will always return 0 when an interface is its right argument.

Interfaces can be compared with ~ and = using reference equality and concatenated into lists using , like any other datatype:


All interfaces will accept the "type" index, which behaves like typeof. This is handy if you ever have a list, dictionary, or table containing interfaces:


Accessing an invalid index will return 0, and attempting to write to an invalid (or read-only) index will return the expression to the right, like any other indexed assignment:


Appendix 1: Unary Primitives

The unary arithmetic primitives - (negation), ! (logical not), floor, cos, sin, tan, exp (the exponential function), ln (natural log), and sqrt (square root) conform, and generalize to both lists and numbers.

The unary aggregation primitives sum, raze, min, and max take a list and collapse it into a single result as if by combining every element of the list with the binary primitives +, ,, & and |, respectively.

typeof gives the name of the type of the argument; one of the strings { "number", "string", "list", "dict", "table", "function" }, or, in the case of an interface, the name of that interface type.

count gives the number of elements in a value. The count of a number is always 1.

first and last extract the first or last elements of a value.

range of a number produces a list of the integers [0,x). Applied to anything else, it will produce a list of the keys of its dictionary equivalent.

list of anything produces a list of length 1 containing that value.

flip will transpose the elements of a list of lists- swap the x and y axes. For example, flip (list 1,2,3),(list 4,5,6) is ((1,4),(2,5),(3,6)). The result will always be rectangular: sublists that are too short will be padded with 0, and any non-list values will be spread to take up an entire column in the result. Applied to a table x, flip will produce a new table using a column named key (or the first column) as new column keys, and all other columns will become rows. The original keys will become a column named key in the resulting table. This operation can be useful for “pivoting” categorical data.

rows converts a table into a list of dictionaries, each representing a row of the table.

cols converts a table into a dictionary of uniform-length lists, each representing a column of the table.

table constructs a table. Given a list-of-dictionaries (as produced by rows) or a dictionary, it will reconstitute a table, taking the union of any dictionary keys as columns and extending any short columns as with take. A list of lists will be interpreted as a list of rows, padded to rectangularity with 0 if needed, using default column names {c0, c1cn}. A list containing non-dictionary and non-list values will be converted into a table with a single value column.

mag computes the magnitude of a vector, or the euclidean distance between a point and the origin. If its argument is anything except a number or list of numbers it will descend recursively through the argument before computing a magnitude, similar to how other unary primitives conform. For example, mag ((list 9,0),(list 3,4),(list 0,7)) is (9,5,7). The magnitude of a single number is its absolute value.

heading computes the angle in radians from the origin toward a point. This is similar to the atan2(y,x) quandrant-disambiguating arctangent provided in many math libraries. Like mag, heading will descend recursively through any non list-of-number arguments, so it can be applied to points, lists of points, and so on.

unit computes a unit vector (with mag of 1) pointing toward an angle in radians. This operation conforms. If the angle is between 0 and 2 pi and the original mag was 1, unit is the inverse of heading.

Appendix 2: Binary Primitives

The binary arithmetic and comparison primitives +, -, *, /, % (modulus), ^ (exponentiation), <, >, =, & (minimum), and | (maximum) conform, and generalize to both lists and numbers.

The modulus operator % takes its arguments in the opposite order of common notation- the divisor is the left argument. Thus, 5 % 3,4,5,6,7 is (3,4,0,1,2). In common usage, this order will require fewer parentheses given Lil’s right-to-left precedence rule.

The <, > and = comparison operators always produce the number 1 or 0 as a result. If both operands are a number, they are compared numerically. If either is an interface, they are equal only if both are identical. Otherwise, the arguments are treated as strings and compared lexicographically.

The & and | operator calculate the minimum or maximum of their arguments. As a consequence, for the numbers 0 and 1 they are equivalent to logical “AND” and logical “OR”- thus their notation. Applied to strings, they compare values lexicographically in the same fashion as < and >.

x ~ y is the match operator. It produces the number 1 if x and y are identical values. Unlike =, match does not convert arguments or automatically “spread” to list elements; x and y must have identical types to begin with. This is particularly important if you want to e.g. check whether an item is the empty list: ()=1,2 yields (), but ()~1,2 yields 0.

x , y is the concatenation operator. It is used for joining items together into lists. If x is a dictionary, y will be converted to a dictionary, and the operator will take the union of their key-value mappings, preferring any bindings in y over x when both are present. If applied to two tables, their rows will be concatenated, with any missing columns supplied as 0. Note that using , on two strings will result in a list of two strings.

x @ y is spread-indexing: x is indexed with each element of y. For example, (11,22,33) @ 0,1,0,1,0 is (11,22,11,22,11). With an appropriate index y, this operator can be used to reorder, duplicate, or filter elements of a list x. The expression x @ y is essentially equivalent to each v k i in y (x[v k i]) end, and so it can also be used as shorthand for any each loop that would otherwise simply be applying a function to each element of its source.

x split y breaks a string y apart on any instance of the string x, resulting in a list of strings.

x fuse y conversely combines the strings in the list y with the string x, resulting in a string. Combining split and fuse can allow one to replace instances of one substring with another.

x dict y constructs a dictionary from a list of keys x and a list of values y.

x take y and x drop y are very general operators for filtering and reshaping data. Their behaviors depend on the type of the left and right arguments, as summarized in the table below. When x is a number, take will repeat elements from y if it isn’t long enough. For non-numeric x values, take can be thought of as set intersection, while drop can be thought of as set difference.

x y take drop
number string first x characters from y remove first x characters from y
number list first x elements from y remove first x elements from y
number dict first x key/value pairs from y remove first x key/value pairs from y
number table first x rows from y remove first x rows from y
negative number string last x characters from y remove last x characters from y
negative number list last x elements from y remove last x elements from y
negative number dict last x key/value pairs from y remove last x key/value pairs from y
negative number table last x rows from y remove last x rows from y
non-number/list string only characters in x of y remove instances of characters in x from y
non-number/list list only elements in x of y remove instances of elements in x from y
non-number/list dict only keys in x of y remove keys in x from y
string table only column x of y remove column x from y
string list table only columns in x of y remove columns in x from y
number list table only rows x of y, in order remove rows in x from y

x limit y returns up to x items from y. Equivalent to if x<count y x take y else y end.

x in y returns the number 1 or 0 depending upon whether y appears in x. If y is a string, look for the string x anywhere in y. If y is a list, look for x as an element of y. If y is a dictionary, x must be a key of that dictionary. If y is a table, it must likewise be a key (column name) of that table. In all other cases, in returns 0. If x is a list, consider each element of x and return a list of 1 or 0.

x unless y returns x unless y is not the number 0; otherwise it returns y. This “null-coalescing operator” is handy for providing default values when indexing into dictionaries or lists. For example, () unless foo[x] will evaluate to the empty list if foo contains no value with the key x.

x join y performs a natural join upon the tables x and y. The intersection between column names in x and y are used as the matching key. For any combination of rows in x and y where the matching key is identical, the result will contain a row containing the union of the values in that row. The resulting table’s column names will be the union of the names in x and y. Natural joins neatly capture foreign-key relationships between tables, provided that column names are consistent. If join is applied to non-table arguments, it produces a list by pairing adjacent elements from x and y. If either argument is a number, it is considered range of that argument, and otherwise it is interpreted as a list. For example, "ABC" join 3 gives (("A",0),("B",1),("C",2)).

x cross y performs a cartesian product (or cross join) of the tables x and y. Every row from x is concatenated with every row in y, producing (count x)*(count y) resulting rows. The resulting table will contain all the columns of both x and y, with any collisions in y gaining an underscore (_) suffix to disambiguate. A cross product filtered by a query can be used to perform inner joins. If cross is applied to non-table arguments, it produces a list of all pairings of elements from x and y. If either argument is a number, it is considered range of that argument, and otherwise it is interpreted as a list. Thus, 2 cross "ABC" gives ((0,"A"),(1,"A"),(0,"B"),(1,"B"),(0,"C"),(1,"C")).

x parse y breaks apart a string or list of strings y based on a format string x. x format y constructs a formatted string from a value or list of values y based on a format string y. The parse and format primitives are generally (but not strictly) inverses, given the same format string.

Appendix 3: Language Grammar

The following is a slightly hand-waved EBNF description of Lil’s syntax. The production ALPHA is taken as any alphabetic character (upper- or lowercase), and the production NON_ESC is any ASCII character between 32 and 126 (except backslash). All whitespace is interchangeable in Lil, any amount of whitespace may appear between tokens, and line comments begin with #.

MONAD   := '-'|'!'|'floor'|'cos'|'sin'|'tan'|'exp'|'ln'|'sqrt'|'count'|'first'|'last'
DYAD    := '+'|'-'|'*'|'/'|'%'|'^'|'<'|'>'|'='|'&'|'|'|','|'~'|'@'|'split'|'fuse'|'dict'|'take'|'drop'|
DIGIT   := '0'|'1'|'2'|'3'|'4'|'5'|'6'|'7'|'8'|'9'
NUMBER  := '-'? DIGIT+ '.'? | DIGIT* '.' DIGIT+
STRING  := '"' (NON_ESC|'\\'|'\"'|'\n')* '"'
NAME    := (ALPHA|'_'|'?') (ALPHA|'_'|'?'|DIGIT)*
ITER    := (( each' NAME* 'in' ) | 'while') EXPR* 'end'
ON      := 'on' NAME+ 'do' EXPR* 'end'
IF      := 'if' EXPR* ( 'else' EXPR* )? 'end'
QUERY   := ('select'|'extract'|'update')((NAME|STRING ':')?EXPR)*('where'EXPR)?('by'EXPR)?('orderby'EXPR('asc'|'desc'))?'from' EXPR
INSERT  := 'insert' (NAME|STRING ':' EXPR) 'into' EXPR
SEND    := 'send' NAME '[' EXPR* ']'
INDEX   := ('.' NAME? | '[' EXPR* ']')*
EXPR    := TERM ( INDEX ( ':' EXPR )? | DYAD EXPR )?