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@sinelaw sinelaw/tf.hs
Created Feb 19, 2015

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{-# OPTIONS -Wall #-}
{-# LANGUAGE TypeFamilies, FlexibleInstances #-}
import Data.Map
class Indexable a where
type Index a
type Result a
index :: a -> Index a -> Result a
instance Indexable [a] where
type Index [a] = Int
type Result [a] = a
index = (!!)
newtype Str = Str { getStr :: String }
instance Indexable Str where
type Index Str = Int
type Result Str = Char
index = (!!) . getStr
instance Indexable (Map String a) where
type Index (Map String a) = String
type Result (Map String a) = a
index = (!)
f :: (Indexable a, Index a ~ Int) => a -> Result a
f x = index x (0::Int)
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