[Haskell-cafe] Type families are awesome

Luke Palmer lrpalmer at gmail.com
Wed Jan 21 03:35:53 EST 2009

2009/1/21 John Ky <newhoggy at gmail.com>
> *Main> let x = lookup
> *Main> let y = Fx.Data.Map.lookup
> <interactive>:1:8:
>     Ambiguous type variable `ma' in the constraint:
>       `Fx.Data.Map.MapType ma'
>         arising from a use of `Fx.Data.Map.lookup' at <interactive>:1:8-25
>     Probable fix: add a type signature that fixes these type variable(s)

I think this is just the monomorphism restriction. Turn it off with
-XNoMonomorphismRestriction, or add a parameter to the binding:

let x k = lookup k

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