Hi Merijn,
Let me suggest the Fail type family in
http://www.haskell.org/pipermail/haskell-cafe/2013-November/111549.html
--
Adam
On Tue, Jan 14, 2014 at 8:56 AM, Merijn Verstraaten
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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