[GHC] #12162: Concatenation of type level symbols missing

[GHC]

#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 RyanGlScott):

 I'm indifferent about what to name `(<>)`, but the fact that someone has
 objected to it means it might be worth starting a thread on the
 [https://mail.haskell.org/pipermail/libraries/ libraries mailing list] to
 figure out the community's opinion. I've heard convincing arguments for
 both `(<>)` and `(++)`. Or `(+++)`, which is what the
 [https://github.com/konn/ghc-typelits-
 symbols/blob/cd812f4cfc2e6816a18283a6a0e9bb2d9ea2905e/src/GHC/TypeLits/Symbols/Internal.hs#L6-L8
 ghc-typelits-symbols] GHC plugin library has taken to calling this
 operation. Or `AppendSymbol`, if we really want to explore all options.

 If you wish to add tests for this feature, there is a portion of the GHC
 test suite located in `base`
 [http://git.haskell.org/ghc.git/tree/HEAD:/libraries/base/tests here].

 And you're absolutely correct in observing that the `Symbol` API is quite
 feature-poor at the moment. OtToMH, there are only three useful operations
 over `Symbol`s in `base`:

 * You can check whether two symbols are equal (either through type-level
 equality or through [http://hackage.haskell.org/package/base-4.9.0.0/docs
 /GHC-TypeLits.html#v:sameSymbol sameSymbol]).
 * You can compare two symbols lexicographically with
 [http://hackage.haskell.org/package/base-4.9.0.0/docs/GHC-
 TypeLits.html#t:CmpSymbol CmpSymbol].
 * You can convert a `KnownSymbol` to a `String` value with
 [http://hackage.haskell.org/package/base-4.9.0.0/docs/GHC-
 TypeLits.html#v:symbolVal symbolVal].

 And that's about it. While it'd be nice to have promoted lists of `Char`s
 and just promote all operations over lists, that's much more work, and
 `Symbol`'s all we've got for now, so we might as well work with it.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12162#comment:11>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler