;; Type predicates

string, integer, ...      ;; 8 built-in types in Lua
{:x integer :y integer}   ;; product, map
{integer true}            ;; product, set
[string]                  ;; product, sequence, homogeneous
[string number]           ;; product, sequence, heterogeneous, fixed length
[:a :b :c]                ;; sum, enumeration, sequence
{:a true :b true :c true} ;; sum, enumeration, set
(fn even? [value] ...)    ;; custom
_                         ;; anything

(type point {:x number :y number})       ;; new type, macro
(assert (supertype? vertex module.node)) ;; interoperability check

;; Sub-type checking.

;; New keyword: &in

(number 1)     ;; evaluates to 1
(number :hi)   ;; type error: `hi' is not a number

;; Type checking at function boundaries.

;; Goal: Avoid identifier-type repetition, encourage reuse.

;; New keyword: &to

(point origin)                     ;; check type
{: name : education &in person}    ;; check sub-type, map
[first second &in pair]            ;; check sub-type, sequence

(fn origin? [point &to boolean])                    ;; Automatic types, all
(fn scale [point (number factor) &to point])        ;; Automatic types, some
(fn success? [{: status &in response} &to boolean]) ;; Destructuring, maps
(fn origin? [{: x : y &in point} &to boolean])      ;; Destructuring, maps
(fn first? [head &in list &to boolean])             ;; Destructuring, sequences

;; Automatic typing encourages:
;;
;; "It is better to have 100 functions operate on one data structure than 10
;; functions on 10 data structures.  --- Alan Perlis