#12162: Concatenation of type level symbols missing -------------------------------------+------------------------------------- Reporter: augustss | Owner: phadej Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D2632 Wiki Page: | -------------------------------------+------------------------------------- Comment (by ekmett): If we build in {{{#!hs type family (++) :: Symbol -> Symbol -> Symbol }}} that doesn't prevent it from later being used in a potential later or even third-party polykinded {{{#!hs type family (<>) :: k -> k -> k type instance (<>) = (++) }}} and it would be consistent with the fact that the other `GHC.TypeLits` such as `(+)` are all specific to one kind. That said, none of the existing machinery for `KnownNat` knows anything about `(+)`, so it'd be a leap to expect `KnownSymbol` to know about `(++)`. Also note that complicating evidence by giving it more internal hidden constructors like is used to deal with `Coercible` instances is actually bad for folks who use things like reflection to mimic this behavior or construct symbols at runtime today. As an aside, you can actually get type level symbol concatenation and tools for working with it out of the `constraints` package at least in HEAD today, but it doesn't have any compiler assistance. You have to explicitly use the various axioms and `appendSymbol` proofs rather explicitly to get the resulting `KnownSymbol` instances, but you can do it. https://github.com/ekmett/constraints/blob/master/src/Data/Constraint/Symbol... -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12162#comment:13 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler