Re: HsCmdArrForm Fixity

21 Jun 2024

      That looks strange.  I would just blaze ahead and do what seems sensible.

That (Just _) equation was introduced by Daniel Winorgrad-Cort in the
commit below.     You could try to see what that commit did, I suppose.
(Twelve years ago.)

Arrows are sadly un-loved (need a champion) so I think you'll just have to
do your best.

Simon

*commit ba56d20d767f0425f6f7515fa9c78b186589b896*
Author: Simon Peyton Jones 
Date:   Wed Oct 3 11:16:22 2012 +0100

    This big patch re-factors the way in which arrow-syntax is handled

    All the work was done by Dan Winograd-Cort.

    The main thing is that arrow comamnds now have their own
    data type HsCmd (defined in HsExpr).  Previously it was
    punned with the HsExpr type, which was jolly confusing,
    and made it hard to do anything arrow-specific.

    To make this work, we now parameterise
      * MatchGroup
      * Match
      * GRHSs, GRHS
      * StmtLR and friends
    over the "body", that is the kind of thing they
    enclose.  This "body" parameter can be instantiated to
    either LHsExpr or LHsCmd respectively.

    Everything else is really a knock-on effect; there should
    be no change (yet!) in behaviour.  But it should be a sounder
    basis for fixing bugs.

On Thu, 20 Jun 2024 at 19:51, Alan & Kim Zimmerman 
wrote:
...
We are currently wrapping up
https://gitlab.haskell.org/ghc/ghc/-/merge_requests/12842, which moves
Fixity declarations into TTG extensions point, and into the haskell syntax,
not GHC.
But rnCmd for HsCommandArrForm in current master has
rnCmd (HsCmdArrForm _ op _ (Just _) [arg1, arg2])
  ...
It matches on HsCmd GhcPs, which never introduce the `Just` form for
fixity.
The only place the `Just` form is introduced, is in mkOpFormRn, but this
is only ever called from this specific equation.  So both can be removed,
without changing behaviour.
The question is, this code was added for a reason at some point, what has
broken in the meantime?  Should this be fixed somehow instead of being
removed?
Alan
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