[Haskell-cafe] Re: Rank-2-polymorphism problem
apfelmus at quantentunnel.de
Sat Mar 24 04:22:22 EDT 2007
Martin Huschenbett wrote:
> readValue :: Field -> (forall s. SqlBind s => s) -> Value
> readValue _ = ...
> That works just fine. But now I want a version of readValue that has a
> Maybe wrapped around the second parameter and that shall call readValue
> in the case of a Just and emptyValue in the case of Nothing. But I can't
> figure out how to write this function as I always get compiler errors.
> -- The type I want to get.
> readValue' :: Field -> (forall s. SqlBind s => Maybe s) -> Value
> -- Fourth trial:
> readValue fld s = case s of
> Just s' -> readValue fld s'
> Nothing -> emptyValue fld
For me, the fourth trial works, at least on
f :: (forall s . Num s => Maybe s) -> Int
f y = case y of
Just x -> x
Nothing -> 0
Your definition has a typo (missing tick '), maybe that's the cause why
it doesn't work?
Furthermore, I guess that let-binding any troublesome rank-2-variables
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