#11391: TypeError is fragile -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: high | Milestone: 8.0.1 Component: Compiler | Version: 8.0.1-rc1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider this use of the new `TypeError` feature, {{{#!hs {-# LANGUAGE TypeInType, TypeFamilies, UndecidableInstances #-} {-# LANGUAGE UndecidableInstances #-} import Data.Kind import GHC.TypeLits (TypeError, ErrorMessage(..)) type family Resolve (t :: Type -> Type) :: Type -> Type where Resolve _ = TypeError (Text "ERROR") }}} ==== The okay case ==== Given something like, {{{#!hs testOK :: Resolve [] Int testOK = [] }}} we find that things work as expected, {{{ • ERROR • In the expression: [] :: Resolve [] Int In an equation for ‘testOK’: testOK = [] :: Resolve [] Int }}} This is what we would expect. ==== The bad case ==== However, it is very easy to fool GHC into not yielding the desired error message. For instance, {{{#!hs testNOTOK1 :: Resolve [] Int testNOTOK1 = () }}} gives us this a unification error, {{{ • Couldn't match type ‘()’ with ‘(TypeError ...)’ Expected type: Resolve [] Int Actual type: () }}} This clearly isn't what we expected. ==== The tricky case ==== Another way we can fool the typechecker is to make it attempt instance resolution on our `TypeError`, {{{#!hs testNOTOK2 :: Resolve [] Int testNOTOK2 = 1 }}} Which results in, {{{ • No instance for (Num (TypeError ...)) arising from the literal ‘1’ • In the expression: 1 In an equation for ‘testNOTOK2’: testNOTOK2 = 1 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11391 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler