[Haskell-cafe] is monomorphism restriction necessary?

[Alexey Muranov]

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 
> convenience? 
> > 
> > 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 
> signature 
> > 
> >     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. 
>
> Roman 

  
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 
this example?

Alexey.
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