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jkarni/show.hs

Last active September 3, 2015 00:32
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show.hs
{-# LANGUAGE ConstraintKinds #-}
module Main where
import Data.Constraint
show' :: (Maybe (Dict (Show a))) -> a -> String
show' (Just Dict) x = show x
show' Nothing _ = "<<not showable>>"
t1 = show' (Just Dict) 5
-- "5"
t2 = show' Nothing id
-- "<<not showable>>"
-- Doesn't typecheck
-- t3 = show' (Just Dict) id
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