[ghc-steering-committee] #216: Qualified Do again, recommendation: accept the alternative

Simon Peyton Jones simonpj at microsoft.com
Mon May 11 08:12:22 UTC 2020

I agree that having longer names would mean that you may have to say

module M( (>>=), (>>), fail

             , qualifiedBind, qualifiedThen, qualifiedFail ) where

qualifiedBind = (>>=)

qualifiedThen = (>>)

qualifiedFail = fail

…defns of (>>=) etc…

but you could regard those extra lines as a feature, rather than a bug. They say “I’m planning to use these operators for M.do”.   I like that.  And I like being able to grep for things in a large codebase.  (Grepping for (>>=) is useless.)

But I can see that judgements might vary.


From: ghc-steering-committee <ghc-steering-committee-bounces at haskell.org> On Behalf Of Alejandro Serrano Mena
Sent: 09 May 2020 20:13
To: Joachim Breitner <mail at joachim-breitner.de>
Cc: ghc-steering-committee at haskell.org
Subject: Re: [ghc-steering-committee] #216: Qualified Do again, recommendation: accept the alternative

I am with Joachim in the use of "normal" names as opposed to special "qualifiedX". At least in my case, I tend to use >>= and >> here and there in my monadic code, so having them exported from the same module would be desirable. In fact, I would expect those modules to also export something akin to <$> and <*>.


El sáb., 9 may. 2020 a las 0:58, Joachim Breitner (<mail at joachim-breitner.de<mailto:mail at joachim-breitner.de>>) escribió:
Hmm indeed :-)

Both ways are justifiable. Allow me to explain why using the normal
operations seem to be a better trade off.

Here is an hypothesis:

  Most people using do-notation throw in some (non-sugar) monad
  operations from time to time, e.g.:

     do doFoo
        z <- bar x >>= baz >>= quux
        return (f z)

This will not change if they use qualifed monads, so we better allow
them to write

     M.do doFoo
        z <- bar x M.>>= baz M.>>= quux
        M.return (f z)

So I expect that any module M that is set up to be used qualified will
want to export (>>=) and (>>) and return anyways.

This observation does not require us to use these in the desugaring.
But since they are there, it seems natural to use them.

If we don’t, then
 * some modules export qualifiedDoBind and (>>=)
   which is redundant (and makes one wonder if they are the same)
 * a few modules might not export (>>=) (only qualifiedDoBind)
   for whatever reason
   This adds cognitive load on the user, who now has to check and
   remember for which M.do they also can use  M.>>=

I acknowledge the downside that such a module M may be less easy to
recognize as “supports qualified do” just given the list of exports.
(Some may call that a feature, e.g. Prelude.do would just work…)
I hope that a good haddock module header can address that, maybe by
having a section “Used of the Linear monad with QualifiedDo” or such.


Am Freitag, den 08.05.2020, 22:40 +0000 schrieb Simon Peyton Jones:
> Hmm.  I can see the attraction of using other less widely used names like
>       M.qualifiedDoBind
>       M.qualifiedDoFail
>       M.qualifedDoMFix
> Then searching for `qualifiedDo` would find the things that are used
> for, well qualified do.  Remember, by using the module-prefix form we
> are making it harder to group together the operations that make up
> the things used by qualified do -- (>>), (>>=) etc are among the most
> widely used lexemes in Haskell.
> I'm not immovable on this, but I think there's a case for using long, explicit names.  If you end up saying
>       qualifiedDoBind = (>>=)
> that would be a very good clue that you intend this operation to be used by Qualified Do.
> Simon
> > -----Original Message-----
> > From: ghc-steering-committee <ghc-steering-committee-bounces at haskell.org<mailto:ghc-steering-committee-bounces at haskell.org>>
> > On Behalf Of Joachim Breitner
> > Sent: 08 May 2020 23:02
> > To: ghc-steering-committee at haskell.org<mailto:ghc-steering-committee at haskell.org>
> > Subject: Re: [ghc-steering-committee] #216: Qualified Do again,
> > recommendation: accept the alternative
> >
> > Hi,
> >
> > Am Mittwoch, den 06.05.2020, 16:28 +0200 schrieb Joachim Breitner:
> > > Hi,
> > >
> > > Am Mittwoch, den 06.05.2020, 15:55 +0200 schrieb Spiwack, Arnaud:
> > > > There is one question to solve: should we use the standard names
> > > > `(>>=)`, `(>>)` for desugaring? (so that the type class methods can
> > > > be used directly). Or some dedicated names `desugaringBind`, … ? To
> > > > avoid name clashes.
> > >
> > > given that the recommended idiom is to only use this with a qualified
> > > module name, I think using the normal, well-known names is reasonable.
> >
> > do we have more opinions on this? If not we can go with the author’s
> > proposal, which is to use the standard names. It’s natural that when I can
> > write `M.do { a M.>> b ; c }` after all, and helpful if programmer can
> > expect M.>> to be there for every module M that they would use to qualify
> > `do`.
> >
> >
> > Cheers,
> > Joachim
> > --
> > Joachim Breitner
> >   mail at joachim-breitner.de<mailto:mail at joachim-breitner.de>
> >   http://www.joachim-breitner.de/<https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.joachim-breitner.de%2F&data=02%7C01%7Csimonpj%40microsoft.com%7Ce9fed952b2ac4d6d94e808d7f44d0197%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637246483894365128&sdata=aVA%2BFFcNViULzX%2B1awe8oCkE74avIhz50jyOup0tYAs%3D&reserved=0>
> >
> >
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