Re: [GHC] #14293: View patterns with locally defined functions in restructuring don't compile

14 Nov 2018

      #14293: View patterns with locally defined functions in restructuring don't compile
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        Reporter:  heisenbug         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  ViewPatterns
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #15893            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 I've realised that it may be simpler than I thought.

 Currently `RnBinds.rnLocalValBindsLHS` looks like this
 {{{
 rnLocalValBindsLHS fix_env binds
   = do { binds' <- rnValBindsLHS (localRecNameMaker fix_env) binds

        ; let bound_names = collectHsValBinders binds'
        ...
 }}}
 I think instead we can do this
 {{{
   = do { let bound_rdr_names = collectHsValBinders binds'
        ; bound_names <- mapM newLocalBndrRn bound_rdr_name
        ; binds' <- bindLocalNames bound_names           $
                    addLocalFixities fix_env bound_names $
                    rnValBindsLHS (localRecNameMaker fix_env) binds

        ...
 }}}
 Take an tricky example binding
 {{{
     (f, f -> x) = e
 }}}
 The idea is

 1. Find all the `RdrName` binders from bindings, here `f` and `x`.

 2. Make fresh `Names` for each of them, say `f_34` and `x_88`.

 3. Extend the environment with bindings `f :-> f_34` and `x :-> x_88`.

 4. Then rename the LHSs; but when we find a binder (say the `RdrName`
 `f`),
    `localRecNameMaker`  should ''look it up'' in the environment, rather
 than making
    a fresh `Name` (that's been done already).

    Result is the pattern `(f_34, f_34 -> x_88)`.

 5. After that, everything is as before.

 To do this we need to persuade `localRecNameMaker` to look up rather than
 make a fresh `Name`.
 I think we can do this easily, by altering the `LetMk` case of
 `newPatName`.

 We need to make exactly the same change in `rnSrcDecls`, for the same
 reason.

 I think that's it!

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14293#comment:4
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