[Haskell-cafe] is monomorphism restriction necessary?
alexey.muranov at gmail.com
Sat Apr 16 19:22:04 UTC 2016
On Saturday, April 16, 2016 at 5:49:24 PM UTC+2, Roman Cheplyaka wrote:
> On 04/16/2016 02:53 PM, Alexey Muranov wrote:
> > Sorry for the question that probably is already answered somewhere, but
> i do not get it from the basic documentation i have looked at so far.
> > Is monomorphism restriction really necessary, or is it just a
> > In the example from A History of Haskell
> > genericLength :: Num a => [b] -> a
> > f xs = (len, len)
> > where
> > len = genericLength xs
> > can't the monomorphism restriction be replaced with just adding the
> > f :: Num a => [b] -> (a, a)
> > ?
> No; the let binding 'len' would still generalize. It would generalize
> even if you assigned 'f' a monotype such as [b] -> (Int, Int).
> Now, the compiler *could* later realize that the polymorphic value 'len'
> is evaluated twice at the same type and perform a CSE, but I guess the
> standard authors were not comfortable with *requiring* that kind of
> optimization from all compliant compilers.
> Of course, nowadays we have scoped type variables, so MR is no longer
> strictly necessary. Sometimes it is desirable in that it gives the
> behavior that you would intuitively expect; sometimes, the opposite.
Thank you, i will think about it, though i don't understand yet.
Could you show me, please, how to use scoped type variables to avoid MR in
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Haskell-Cafe