[GHC] #14527: Warn on recursive bindings

[GHC]

#14527: Warn on recursive bindings
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        Reporter:  chrisdone         |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      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:                    |
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Comment (by nh2):

 **GHC 7.10 already had this feature**, but it disappeared with 8.0, and
 it's not clear to me if intentionally or accidentally. It would be great
 to find out.

 In 7.10, you could simply used a BangPattern to forbid recursive variable
 bindings. Example

 {{{
     Recursive bang-pattern or unboxed-tuple bindings aren't allowed:
       !x = x
 }}}

 And it said in the manual:

 {{{
     a bang-pattern binding must be non-recursive
 }}}

 It was removed for GHC 8.0 in these commits:

 *
 https://github.com/ghc/ghc/commit/e7985ed23ddc68b6a2e4af753578dc1d9e8ab4c9
 #diff-47b9f40b0ede605e7b6bcb330b43ad53L1678
 * https://github.com/ghc/ghc/commit/46a03fbec#diff-
 02e7842997523ff8a5ad0312df2edfe2L10348

 The question is: Why was this removed?

 Was it accidental, or did people find a legitimate use case for strict
 recursive variable let bindings?

 If not, we could simply re-introduce that, without a language extension
 (though we might still add the proposed language extension to get warnings
 even without bangs).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14527#comment:3>
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