(I am assuming that also `->` classifies type families although, as you

#6018: Injective type families -------------------------------------+------------------------------------- Reporter: lunaris | Owner: jstolarek Type: feature request | Status: new Priority: normal | Milestone: 7.12.1 Component: Compiler | Version: 7.4.1 Resolution: | Keywords: Operating System: Unknown/Multiple | TypeFamilies, Injective Type of failure: None/Unknown | Architecture: Blocked By: | Unknown/Multiple Related Tickets: #4259 | Test Case: | Blocking: | Differential Revisions: Phab:D202 -------------------------------------+------------------------------------- Comment (by goldfire): Replying to [comment:109 jstolarek]: point out, this is not entirely accurate.) Right -- `->` does '''not''' classify type families in Haskell today. The only place where `->` describes a type family is in the output of GHCi's `:kind` operator. And, to get that to work, GHC has a gross hack in that GHCi relaxes the "type families must always be saturated" requirement, just to appease users who want to know the shape of their type families. (This gross hack is a great idea from a user's point of view, and I'm not at all suggesting removing it. But it is gross from a type system point of view.) Nowhere in a Haskell source text can `->` classify a type family. So, I think that fact describes the difference between our comments above. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/6018#comment:110 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler