#6018: Injective type families -------------------------------------+------------------------------------- Reporter: lunaris | Owner: jstolarek Type: feature | Status: new request | Milestone: 7.10.1 Priority: normal | Version: 7.4.1 Component: Compiler | Keywords: TypeFamilies, Resolution: | Injective Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: | Blocked By: None/Unknown | Related Tickets: #4259 Test Case: | Blocking: | Differential Revisions: Phab:D202 | -------------------------------------+------------------------------------- Comment (by jstolarek): I've made a major update to the wiki page. Here are a few things worth pointing out: In [wiki:InjectiveTypeFamilies#Proposedsyntax this section] I've listed all the syntax proposals made so far. I've also [http://www.haskell.org/pipermail/haskell-cafe/2014-September/116065.html mailed Haskell-cafe] so we can get some feedback from the community. I christened various flavours of injectivity as A, B and C :-) -- see [wiki:InjectiveTypeFamilies#Formsofinjectivity this section]. I will refer to them as such in the rest of this comment. I mailed Conal Elliot and he says he does not remember what was his use case for type families injective in only one argument. This means we only have use cases of the form A shown by Simon. Given that, I believe that it is best if we limit ourselves to implementing A. We can revise this design later if someone demonstrates compelling examples of B or C. I have outlined my idea for the algorithm that would verify users injectivity declarations in [wiki:InjectiveTypeFamilies#Implementationoutline this section]. Simon, please tell me if there are any misconceptions about how GHC's type inference works. After some thinking I am against inferring injectivity for closed type families - see my arguments in [wiki:InjectiveTypeFamilies#Inferringinjectivity this section]. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/6018#comment:67 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler