Commutative function in Haskell

I want to write a function in haskel开发者_StackOverflow中文版l which would not mind in what order I provide it its argument, for example, I want to unify these two functions

    reproduce1 :: Male -> Female -> Child
    reproduce2 :: Female -> Male -> Child

by one function 'reproduce'.

You can do this using a multi-parameter type class.

{-# LANGUAGE MultiParamTypeClasses #-}

class Reproduce x y where
  reproduce :: x -> y -> Child

instance Reproduce Male Female where
  reproduce = reproduce1

instance Reproduce Female Male where
  reproduce = reproduce2

However, I'm curious about why you would want to do this.

Maybe you'd like to package your arguments into a datatype and use records (see "Labelled Fields") instead?

   data Args = A { m :: Male , f :: Female}
   reproduce :: Args -> Child

However, I share @hammar's curiosity.

I was thinking about something like this, which throws an exception if both adults are of the same sex:

module Main where

main = putStrLn (reproduce (Male "a") (Female "b"))

type Child = String
data Adult = Male String | Female String
  deriving (Show)

reproduce :: Adult -> Adult -> Child
reproduce (Male a) (Female b) = a ++ "+" ++ b
reproduce (Female a) (Male b) = b ++ "+" ++ a

I strongly recommend fixing the order, say first Male and then Female, or making a "marriage" datatype as in ShiDoSi's solution.

However, check section "Session types and duality", pg 12 in the paper "Fun with type functions" - I think that is a good example where you need types coupled in symmetric pairs male-female.