Language Tour

Play Along

If you'd like, start up a PiLisp REPL and evaluate expressions as we go.

PiLisp supports a large subset of Clojure's syntax. Clojure is a Lisp, but with fewer parentheses than most, because Rich Hickey took advantage of other delimiters like square brackets and curly braces.

1    ; number
1.1  ; also number, but it floats!
"a"  ; string
:a   ; PiLisp calls these "terms" rather than "keywords"
.a   ; A term can be written using a dot. Dot or colon, makes no difference.
()   ; A persistent list
[]   ; A persistent vector
{}   ; A persistent hash map
#{}  ; A persistent set

Before we go any further, we need to get off the tour bus and talk about the pl> macro.

The pl> macro

When you start the PiLisp REPL, you're presented with the prompt pl>. This is actually a hint that the REPL is calling PiLisp's pl> macro with your code as its arguments.

The pl> macro makes PiLisp a little less "Lispy". It makes top-level parentheses optional, and tries to invoke what you pass it, rather than simply evaluating it.

Compare the following Clojure and PiLisp programs to gain a better understanding:

Clojure

user> rand-int
#object[clojure.core$rand_int 0x2a551a63 "clojure.core$rand_int@2a551a63"]

PiLisp

pl> rand-int 100
42

pl> rand-int 100
73

Now we can get back to the language tour.

Functions

PiLisp is a functional Lisp. For one, this means it treats functions as values.

A lot of things are invocable in PiLisp.

Global Functions

Since PiLisp does not provide namespaces or any kind of module facility, all functions that are bound to names in the environment are global.

You can define a global function using the def special form or defn macro:

(defn point [x y] {.x x .y y})

You can invoke the function either with parentheses or, if you're at the REPL or using the pl> by hand, without parentheses:

(point 2 3)
;;=>
;; {
;;   :x 2,
;;   :y 3,
;; }

;; At REPL
pl> point 2 3
;;=>
;; {
;;   :x 2,
;;   :y 3,
;; }

Notice two additional things:

1. Commas are whitespace. We didn't have to use them when defining point.
2. Terms can be written with either a leading colon or dot, but they're always printed with the colon.

Evaluate (bindings) to see all of the bindings in the current PiLisp environment. You can further refine this to functions:

(defn fn-binding? [binding-entry]
  (let [[symbol binding] binding-entry]
    (fn? (:value binding))))
;;=> #<function: fn-binding?>

(count (filter fn-binding? (bindings)))
;;=> 878

Functions, however, aren't the only invocable data types.

Invocable Data Types

Terms and strings will look themselves up in associative collections:

pl> (.a {.a "alpha" .b "beta"})
"alpha"

pl> ("name" {"name" "PiLisp" "host" "Dart"})
"PiLisp"

Collections are functions: vectors of their indices, maps of their keys, sets of their members:

([.a .b .c .d] 2)
;;=> :c

({.alpha "a" .beta "b"} .beta)
;;=> "b"

(#{"I" "you" "we" "y'all"} "y'all")
;;=> "y'all"