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Jonplussed/haskell-default-args.hs

Last active August 29, 2015 14:21
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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
-}
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