As i understand it, the monomorphism restriction applies to "constrained type variables", but c :: forall a. a, so it remains polymorphic and each of its uses can be instantiated to a different type.
2007/12/25, Henning Thielemann <lemming@henning-thielemann.de>:
I thought I understand monomorphism restriction. But it seems, I don't. I
have boilt down my problem to the following test. Don't try to make any
sense of it, it is just the smallest code I could come up with.
test :: (Integral a, RealFrac a) => a
test =
let c = undefined
in asTypeOf (round c) c
When compiling I get:
Compiling StorableInstance ( src/StorableInstance.hs, interpreted )
src/StorableInstance.hs:38:17:
Warning: Defaulting the following constraint(s) to type `Double'
`RealFrac a' arising from use of `round' at src/StorableInstance.hs:38:17-21
In the first argument of `asTypeOf', namely `(round c)'
In the definition of `test1': test1 = let c = undefined in asTypeOf (round c) c
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