PebbLisp

A very basic LISP implementation with an interpreter and editor written for the Pebble.

Writing in PebbLisp

PebbLisp includes several built-ins functionalities.

Def

def stores a given object into the environment with the given symbol. The general form is:

(def symbol object)

As an example, to store a long greeting message into the short symbol g:

(def g "Hello, how are you today?")

g will then evaluate to "Hello, how are you today".

Remember that defining an object with the same symbol as an object that already exists is possible, but deletes the original object from the environment!

If

if checks a given condition. If the condition evaluates to true, the first given expression is returned. If false, the second expression is returned. The general format is:

(if condition expr1 expr2)

The condition will typically be involve a comparison operator. For example:

(if (< 5 10) "Yee" "haw")

Would return "Yee", as (< 5 10) aka 5 < 10 evaluates to true.

Fn

fn creates a lambda with a (theoretically) arbitrary number of arguments to be evaluated on call. The general form is:

(fn (arg-list) (lambda-body))

A lambda will commonly be stored under a def symbol, for example:

(def sq (fn (a) (* a a)))

Defines a simple lambda to square a given number. Calling it is as simple as (sq 5), which returns 25.

Lambdas can also be applied anonymously, as in ((fn (a) (* a a)) 5), which also returns 25, but this is most useful when using something like map. For example:

(map (fn (a) (* a a)) (1 2 3 5 8 13 21 34))

Uses an anonymous lambda to square each element in the list. This is particularly useful on a low-memory device like the Pebble, where it may be useful to avoid storing the named lambda object in the environment.

Cat

cat returns its arguments concatenated as strings. It has the same general form as arithmetic operators

(cat expr1 expr2 ...)

For example, combining numbers and strings:

(cat "There are " 5 " cats")

Would return There are 5 cats.

A cat operation is applied implicitly when using + with strings, but this may result in confusing behavior when combined with number objects, due to the left-associativity of operators. For example, (+ 5 10 " cats") results in 15 cats but (+ "Cats: " 5 10) results in Cats: 510.

Map

map applies a given lambda to each element in a given list and returns a list composed of each result. The general form of a map expression is

(map lambda (input-list))

For example, using a sq lambda:

(map sq (1 2 3 4 5 6 7 8 9 10)

Would return ( 1 4 9 16 25 36 49 64 81 100 ), with each element being the square of the corresponding element in the input list.

Fil

fil returns a filtered list, based on a given list and a given condition. Partial function support in PebbLisp is nowhere near comprehensive, but fil operates on the bare notion that currently exists. The general form of a fil expression is

(fil (partial-condition) (candidate-list))

Each element in the candidate list is compared against the partial condition. If the comparison returns true, it is added to the returned list. For example

(fil (< 100) (20 150 30 200))

would return ( 150 200 ), as no other elements fit the condition (< 100 n).