[Haskell-cafe] is monomorphism restriction necessary?
alexey.muranov at gmail.com
Sat Apr 16 20:23:28 UTC 2016
On Saturday, April 16, 2016 at 10:10:37 PM UTC+2, Roman Cheplyaka wrote:
> On 04/16/2016 10:22 PM, Alexey Muranov wrote:
> > Could you show me, please, how to use scoped type variables to avoid MR
> > in this example?
> All MR does is it prevents a binding from being generalized. Another way
> to achieve
> that would be to give that binding a (monomorphic) type signature. Without
> scoped t.v., you can't give a signature to 'len' while keeping 'f'
> in the return type. With scoped t.v., you can:
> f :: forall a b . Num a => [b] -> (a,a)
> f xs = (len, len)
> len :: a
> len = genericLength xs
Great, thanks for the explanation (i'll think about it). If i have
understood correctly, having scoped type variables, MR is no longer needed
at all then.
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