simple Haskell help needed on #19746

[Iavor Diatchki]

Hello Richard,

Just FYI `gather` doesn't work with `readS_to_P`  (throws an error), so
you'll have to stick to ReadP parsers but, hopefully, that's OK.

-Iavor

On Tue, Apr 27, 2021 at 7:14 PM Richard Eisenberg <rae at richarde.dev> wrote:

>
>
> > On Apr 27, 2021, at 3:32 PM, Sebastian Graf <sgraf1337 at gmail.com> wrote:
> >
> > Hi Richard,
> >
> > Maybe I lack a bit of context, but I don't see why you wouldn't choose
> (3).
> > Extending the lexer/parser will yield a declarative specification of
> what exactly constitutes a GHC_OPTIONS pragma (albeit in a language that
> isn't Haskell) and should be more efficient than `reads`, even if you fix
> it to scale linearly. Plus, it seems that's what we do for other pragmas
> such as RULE already.
>
> (3) is tempting indeed. There are two problems:
>
> A. The code that parses strings isn't actually declarative. See
> https://gitlab.haskell.org/ghc/ghc/-/blob/d2399a46a01a6e46c831c19e797e656a0b8ca16d/compiler/GHC/Parser/Lexer.x#L1965.
> In particular note the comment: "This stuff is horrible. I hates it."
> Evidently written by Simon M in 2003 with the introduction of alex.
>
> B. We need this code outside the lexer, to deal with e.g. :set in GHCi.
>
> > On Apr 27, 2021, at 4:28 PM, Iavor Diatchki <iavor.diatchki at gmail.com>
> wrote:
> >
> > ... gather ...
>
> Aha! That was the magic incantation I needed but did not have. Many
> thanks, Iavor.
>
> The curious can see
> https://gitlab.haskell.org/ghc/ghc/-/merge_requests/5509/diffs?commit_id=a560fcbbc7d4e37c4909385c55839f793b570e68#c1078a9741c11d1e15d4c678b107092790295bb3_308_317
> for the final result.
>
> Thanks!
> Richard
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