[Haskell-cafe] Hidden types and scope

Lana Black lanablack at amok.cc
Wed Jul 10 16:10:35 UTC 2019

On 10/07/2019 16:03, Sandeep.C.R via Haskell-Cafe wrote:
>  >What is the reason case and let expressions are treated differently in 
> this case
> Please check the section " 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.

Thank you! That explains it.

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