API Annotatons in a FunBind

[Alan & Kim Zimmerman]

On Tue, Dec 23, 2014 at 1:59 PM, Simon Peyton Jones <simonpj at microsoft.com>
wrote:

>  If you do this
>
> ·         Please make Match into a record (it ought to be already)
>
Ok

>  ·         Put a Note on the fun_id field, with an example like the one
> you give
>
Ok

>
>
> I’m not really sure if Haskell lets you mix infix and prefix notation for
> the same function definition, as you have done.  The Report is silent on
> this question. I’m inclined to think “no”.
>

>
> Currently a FunBind has a fun_infix field saying whether the definition
> uses infix notation.  That is fine if “no” above.
>
>
>
> If all are prefix or infix, do you still need the new field in Match?
> Would it matter only for operators where you want to know where to put the
> parens?  Even in normal code, how are you distinguishing (+ ) from ( +) or
> whatever?
>


The example I gave is an empirical test, it is currently accepted by GHC.
The function as a whole is infix, it is just the alternate representation
can be used for the individual Match definitions.

There are straightforward rules as to whether a given fun_id needs parens
or backquotes. The problem is that with source-to-source conversions the
original must be reproduced exactly, and the code author has freedom to put
arbitrary whitespace between the surrounding parens/backquotes and the
actual identifier.  Hence the original fun_id is needed as an anchor for
the API annotations.

Also, there is currently no location information for the fun_id for an
infix definition, which can move around depending on the size of the left
operand, and the choices made for whitespace.

In earlier versions of HaRe we were forced to scan through the rich token
stream for a match to pick up this information.

Alan



>
>
> Simon
>
>
>
> *From:* ghc-devs [mailto:ghc-devs-bounces at haskell.org] *On Behalf Of *Alan
> & Kim Zimmerman
> *Sent:* 15 December 2014 21:16
> *To:* ghc-devs at haskell.org
> *Subject:* API Annotatons in a FunBind
>
>
>
> After individual FunBinds have been parsed, they are combined in
> getMonoBind.
>
> In the process, all the original FunBind fun_id's bar one are discarded,
> including its location.
>
> This causes a problem for source-to-source conversions of functions such
> as the following
>
> (&&&  ) [] [] =  []
> xs    &&&   [] =  xs
> (  &&&  ) [] ys =  ys
>
> Where there are compound RdrNames, and each has different spacing.
>
> I am proposing to add a (Maybe (Located id)) to the Match datatype to deal
> with this.
>
>
> data Match id body
>   = Match
>
>         Maybe (Located id) -- fun_id in subsequent function equations
>
>         [LPat id]               -- The patterns
>         (Maybe (LHsType id))    -- A type signature for the result of the
> match
>                                 -- Nothing after typechecking
>         (GRHSs id body)
>
> Is this a problem?
>
> Alan
>
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