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-ext...
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