Feature request: Vacuous/error constraint (related to 7.7 closed type families regression)

Erik Hesselink hesselink at gmail.com
Tue Jan 14 14:13:34 UTC 2014

You can of course produce an infinite set of vacuous constraints using
the Symbol type (or any two non-equal types). For example, here you
could use:

    Restrict a (a ': as) = ("Error" ~ "Oops! Tried to apply a restricted type!")


On Tue, Jan 14, 2014 at 2:56 PM, Merijn Verstraaten
<merijn at inconsistent.nl> wrote:
> I was trying to fix one of my closed type families examples for the new syntax, and run into an unfortunate issue. Due to a kind error in my code one of my constraints was being silently discarded (see ticket: https://ghc.haskell.org/trac/ghc/ticket/8669)
> The main reason I resorted to this (in hindsight) ill-kinded hack is the lack of a vacuous (i.e. never holding) constraint to produce type errors. I would love to be able to explicitly force GHC into realising "this constraint can never hold", if I had the ability to pass along a String/Symbol to be reported to the user, that would be even better.
> Cheers,
> Merijn
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