haskell-default-args.hs

{-# LANGUAGE FlexibleInstances #-}

module DefaultArgs where

{-
Lots 'o languages give you functions with default arguments. For instance, in Ruby:

    def foo(bar, baz=0, quux=1)
      # stuff...
    end

This is a terrible idea. Changing a default arg value can have a ton of cascading
effects. But it is convenient! So, in cases where it'd be really, really nice to have
default args, how can we get those in Haskell?
-}

data Vec4
  = Vec4 { x :: Int, y :: Int, z :: Int, t :: Int }
  deriving (Eq, Show)

class ToVec4 a where
  toVec4 :: a -> Vec4

instance ToVec4 Vec4 where
  toVec4 = id

instance ToVec4 (Int -> Vec4) where
  toVec4 f = f 1

instance ToVec4 (Int -> Int -> Vec4) where
  toVec4 f = f 0 1
  
{-
Examples:

toVec4 $ Vec4 3 2
=> Vec4 3 2 0 1

toVec4 $ Vec4 4 3 2 5
=> Vec4 4 3 2 5
-}