[Haskell-cafe] Hidden types and scope

Sandeep.C.R sandeep at sras.me
Wed Jul 10 16:03:33 UTC 2019

 >What is the reason case and let expressions are treated differently in 
this case

Please check the section " Restrictions section" in

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.

On 10/07/19 9:26 PM, Lana Black wrote:
> A followup question. What is the reason case and let expressions are 
> treated differently in this case? I can see the error message saying 
> about the type variable escaping its scope, but I don't understand how 
> exactly this can happen with let.
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