[GHC] #15679: Use String rather than [Char] where possible

[GHC]

#15679: Use String rather than [Char] where possible
-------------------------------------+-------------------------------------
           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.6.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Try this in GHCi
 {{{
 Prelude> :t "foo"
 "foo" :: [Char]
 }}}
 It would be better to say
 {{{
 "foo" :: String
 }}}
 Why don't we?  Because of this in `TysWiredIn`
 {{{
 stringTy :: Type
 stringTy = mkListTy charTy -- convenience only
 }}}
 That is, where GHC needs `String` is uses `stringTy` which is just
 `[Char]`.

 How to fix?  Two ways:

 1. Make `String` into a "wired-in type".  That's not hard, but it
 increases the number of wired-in types, which is generally undesirable.

 2. Make `String` into a "knonw-key name", and look it up in the type
 environment on the (few) occasions where we need `stringTy`.  That's a
 little harder -- notably `hsLitType` would become monadic -- but not
 difficult.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15679>
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