|data Test = Test { foo :: Int, bar :: Char, baz :: Bool }
|smallPrint t = concatMap (\f -> show $ f t) [foo, bar, baz]
|In this code the list [foo, bar, baz] should have the type [exists a. Show a =>
Test -> a].
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ExistentialQuantification #-}
data EShow = forall a. Show a => EShow a
smallPrint t = concatMap (\f-> case f t of EShow a -> show a) [EShow . foo, EShow . bar, EShow .
baz]
data Test = Test { foo :: Int, bar :: Char, baz :: Bool }
Apart from the extra wrapping, this hardcodes the class. So perhaps
you'd prefer something like
data E t = forall a. E (a->t) a
smallPrint' t = concatMap (\f-> case f t of E show a -> show a) [E show . foo, E show . bar, E show
. baz]
GHC does have existentials (Hugs has them, too, and HBC had them as well?),
but is more conservative about their use than UHC seems to be.
Claus
PS there's also the old standby of applying the functions in the interface
and letting non-strict evaluation taking care of the rest (keeping the
intermediate type implicit, instead of explicitly hidden):
smallPrint_ t = concatMap (\f-> f t) [show . foo, show . bar, show . baz]
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