gistfile1.clj

;; [09:46] < aphyr> hullo
;; [09:54] < aphyr> I'm trying to type an expression like [f g h], where (f (g (h nil))) is valid.
;; [09:55] < aphyr> e.g. something I could apply (comp) to
;; [09:55] < aphyr> But I'm having trouble figuring out how to enforce that each fn should take the next fn's output type

(ns cttest.core
  (:require [clojure.core.typed :as t]))

(t/ann foo (t/All [a b c]
             [[b -> c] [a -> b] [nil -> a] -> c]))
(defn foo [f g h]
  (f (g (h nil))))

(t/ann int->str [Integer -> String])
(defn int->str [x]
  (str x))

(t/ann str->maybe-int [String -> (t/Option Integer)])
(defn str->maybe-int [x]
  (try
    (Integer/parseInt x)
    (catch NumberFormatException _
      nil)))

(defn this-typechecks []
  (foo str->maybe-int int->str (constantly 0)))

(comment
  (defn but-this-fails []
    ;; g's return value does not match f's input
    (foo int->str str->maybe-int (constantly "0")))

  (defn and-so-does-this []
    ;; h's return type does not match g's input
    (foo str->maybe-int int->str (constantly "foo"))))