[Haskell-cafe] Hidden types and scope

[Benjamin Franksen]

Am 10.07.19 um 18:03 schrieb Sandeep.C.R via Haskell-Cafe:
>>What is the reason case and let expressions are treated differently in
> this case
> 
> Please check the section "7.4.5.4. Restrictions section" in
> https://downloads.haskell.org/~ghc/7.6.3/docs/html/users_guide/data-type-extensions.html
> 
> 
> It says,
> 
>>In general, you can only pattern-match on an existentially-quantified
> constructor in a case expression or in the patterns of a function
> definition. The reason for this restriction is really an implementation
> one. Type-checking binding groups is already a nightmare without
> existentials complicating the picture. Also an existential pattern
> binding at the top level of a module doesn't make sense, because it's
> not clear how to prevent the existentially-quantified type "escaping".
> So for now, there's a simple-to-state restriction. We'll see how
> annoying it is.

FWIW, I personally do find the restriction pretty annoying (even though
by now I am quite used to it).

In some cases one can work around it by using pattern guards instead of
let/where, since these are also exempt.

Cheers
Ben