That reads very oddly to me, but how about...? {{{ type family Id a = result | result -> a where }}} You mean that the `:: result` syntax reads oddly? It is already supported by GHC and the nice thing about this proposal is that we can piggyback on

#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): Replying to [comment:69 rwbarton]: the syntax we already have. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/6018#comment:70 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler