#16249: no runtime error for -fdefer-type-errors with TypeError constraint -------------------------------------+------------------------------------- Reporter: guibou | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Hmm. The trouble here is that, at runtime the `TypeError` dictionary is not actually used. I think we get something like this {{{ data Foo t = MkFoo (t -> t) foo :: Foo t -> t -> t foo (MkFoo f) = f $dfFooString :: TypeError (Text "String does not work") -> Foo String -- Arises from the instance $dfFooString d = MkFoo id derr:: TypeError (Text "String does not work") derr = error "String does not work" x :: String -- Haskell source code: -- x = foo "hello" x = foo ($dfFooString derr) "hello" }}} Evaluating `f` does not evaluate `derr`, hence the no-error run. I suppose someone could think about evaluating calls to `TypeError` more eagerly. But I'd be inclined to think that an easier path is to say {{{ instance (TypeError (Text "String does not work")) => Foo String where foo = error "String does not work" }}} I'm sure there is more than could be done to make `TypeError` better. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16249#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler