Update README.md with explanation of basic built-ins.

README.md

@@ -1,2 +1,81 @@
 # PebbLisp
-A very basic LISP implementation with an interpreter and editor written for the Pebble
+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)`.