#15050: ScopedTypeVariables could allow more programs -------------------------------------+------------------------------------- Reporter: nomeata | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.1 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 goldfire): This is a consequence of the fact that a scoped type variable must be a variable, never a type. Another way of saying this is that a type variable brought into scope in a pattern type annotation simply names an existing type variable; it can't, say, bind a type variable to a type. Of course, with type equalities (and therefore GADTs), you can have a type variable `a` and the constraint `a ~ Int`. In this case, you ''can'' bind a scoped type variable to `a`, despite the equality constraint saying that you're essentially binding a scoped type variable to a type. I've personally never liked this design choice around scoped type variables, believing that a pattern-bound type variable should be able to be bound to any type. The lack of ability to do this has actually been a pain point in `singletons` from the beginning, and these examples show that the current behavior isn't even truly self-consistent. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15050#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler