[Haskell-cafe] NoMonomorphismRestriction forces odd situation when using contexts

[Noon Silk]

Hi Albert,

  Thanks for your response (and sorry about my earlier email missing some
content at the end; I rushed out to lunch as I was sending this).

  Your solution works fine. It's better than the option I've listed.


On Wed, Dec 23, 2015 at 7:57 AM, Albert Y. C. Lai <trebla at vex.net> wrote:

> On 2015-12-20 08:01 PM, Noon Silk wrote:
>
>> {-# LANGUAGE NoMonomorphismRestriction #-}
>>
>> data Status = Foo | Bar
>>
>> data Rec m a = Rec {
>>              get    :: m a
>>            , status :: Status
>>            }
>>
>> defRec :: (Monad m) => Rec m a
>> defRec = undefined
>>
>> myRec :: (Monad m) => Rec m a
>> myRec = Rec x y
>>   where
>>       Rec x y = defRec
>>
>
> Why this is an ambiguous-type error is a really long story. But a factor
> is analogous to "show . read". Another factor is that since you turn off
> the monomorphism restriction, there is a type generalization step, and the
> generalizing of y's type is separate from the generalization of x's type.
>
> I have found this solution, it works by connecting types to suppress the
> generalize step:
>
> {-# LANGUAGE NoMonomorphismRestriction, ScopedTypeVariables #-}
> ...
> myRec :: forall m a. (Monad m) => Rec m a
> myRec = Rec x y
>   where
>       Rec x y = defRec :: Rec m a
>
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-- 
Noon Silk, ن

https://sites.google.com/site/noonsilk/

"Every morning when I wake up, I experience an exquisite joy — the joy
of being this signature."
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