map and fmap

Cale Gibbard cgibbard at
Wed Aug 30 08:38:59 EDT 2006

I haven't really been able to follow this entire thread, but I'd just
like to note here that I agree completely with Jón's take here on the
map issue. It's almost embarassing to have to tell people that there
are 3 functions in the basic libraries which do exactly the same thing
up to type signature. The issue with choosing fmap or liftM is even
worse. I usually go with fmap because it's in the Prelude and seems
more general anyway, but then, some monads aren't in Functor so I need
to be careful about creating class contexts with both Monad and
Functor constraints. Please, let's fix this. Nice general interfaces
are good. While we're at it, let's split MonadZero and MonadPlus --
the decision to merge them was not well thought-out, and a lot of
expressive power in type signatures is lost there.

I'm not sure I agree with Jón's earlier sentiment with regards to
taking everything out of the Prelude and requiring the user to import
many separate modules though. :) While the idea of producing as clean
a language as possible is attractive, the most commonly used list,
monad, and IO functionality is pretty nice to have available without
extra imports. It would also be nice to have all the usual instances
handy straight away.

In any case, it's probably a good idea to at least have those things
in separate modules, and then possibly reexported by the Prelude, as
far as structuring things goes.

I think it would be reasonable to expect the Prelude to include enough
things to provide for basic idiomatic Haskell programming, as it
(mostly) does currently. I already tend to import Data.List and
Control.Monad preemptively, but I should probably be more careful and
take note of which things I should be nagging people to move upward.
At the very least, join should be a member of the Monad class, so it
ought to be there. :)

Lists essentially take the place of loops in Haskell, and even in C,
you don't need to do an #include to get 'for'. I think the issue is
basically one of striking a balance between cleanliness of design, and
ability to write quick (1 to 10 line) programs conveniently.

Over time, the standard practice of writing Haskell code changes too.
For example things like the monad instance for ((->) e) (that is, the
lightweight reader monad) are becoming more popular -- to the point
where I'd mostly feel comfortable asking to have that put in the
Prelude. (All the people who hang around in #haskell have likely
picked up a few idioms from the @pl lambdabot module though -- perhaps
we're a biased sample ;)

 - Cale

On 30/08/06, Jon Fairbairn <jon.fairbairn at> wrote:
> I don't really have the stamina to keep up with discussions
> like this. I have a bit more now than the first time round,
> so here's some more...
> On 2006-08-29 at 07:58+0200 "John Hughes" wrote:
> > On the contrary, it seems we had plenty of experience with an overloaded
> > map--it was in the language for two and a half years,
> During which there were fewer users, as you note below.
> > and two language versions. In the light of that
> > experience, the Haskell 98 committee evidently decided
> > that overloading map was a mistake, and introduced fmap
> > for the overloaded version.
> One might say that your experience persuaded the committee
> to do this.
> > Now, this was an incompatible change, and the Haskell
> > committee was always very wary of making such changes--so
> > there must have been a weight of experience suggesting
> > that overloading map really was a mistake.
> For teaching, yes.
> > It wouldn't have been changed on the basis of abstract
> > discussions of small examples. My own bad experiences with
> > list overloading were with monad comprehensions, but
> > others must have had bad experiences with overloaded map
> > also. Given that it's been tried--and tried so
> > thoroughly--and then abandoned, I would be very wary of
> > reintroducing it.
> I don't think you can conclude that from the evidence
> available (ie the link, posted by Ross Paterson, to the
> discussion at the time)
> > We didn't simplify things in Haskell 98 for the sake of
> > it--we simplified things because users were complaining
> > that actually using the language had become too complex,
> > that there were too many corners to stumble on.
> This is where I most heartily disagree. Whatever the
> arguments for and against, what was done was /not/ a
> simplification of the language.
> I cannot see how it can be argued that a language where
> * the functorial map has three names (fmap, liftM and map)
>   at different types
> * and the general functorial map (fmap) can be applied only
>   to some Monads (the ones where an instance has explicitly
>   given)
> is simpler than a language where
> * the functorial map is called map.
> Your argument that teaching the former language is simpler
> is very strong and I don't dispute it, but it is not, I
> think, a reason to require that people who want to use the
> language to have to put up with remembering extra
> complexity.  Once one knows what functors and monads are
> (and no one can call themselves an expert Haskell programmer
> who does not), one should not have to think "does this Monad
> have an instance of Functor, or must I use liftM?" or is
> this function /really/ meant to work only on lists, or can I
> replace map with fmap and get it to work on something else
> (and then find that it requires copying out the whole
> definition because it also uses ++ or something).
> Yes, it makes perfect sense to have
> > mapList = (map :: (a->b) -> [a] -> [b])
> in a prelude somewhere for teaching purposes, but aren't
> people eventually taught that mapList is just a specialised
> version of map, ++ is `mplus` specialised to lists (etc),
> and that one should think in terms of defining operations
> that are as generally useful as possible?
> At which point don't some of them start to wish that they
> could just type ++ instead of mplus? I certainly do. If it
> were just a question of map and fmap, I might agree that the
> cost would outweigh the benefit, but there's a whole swathe
> of functions for which I'd rather see the nicer names used
> for the more general versions, and clumsier ones for the
> versions specialised to lists for teaching purposes.
> We would all benefit from better error messages, but that's
> a different problem.
> > I think we did a good job--certainly, the Haskell
> > community began growing considerably faster once Haskell
> > 98 came out.
> I'm not sure there's a causal relationship there. If the
> growth was anything above linear, it would be growing faster
> later whether or not Haskell 98 had an effect. Even if
> Haskell 98 was the cause, it's far from obvious that this
> particular change was the one that made the
> difference... and if it did, it may not have done so for a
> good reason.  If you make the language easier to understand
> it may well become more popular (there are plenty of awfully
> popular awful languages out there for more or less that
> reason), but if it's at the expense of unnecessarily complex
> programmes, we shouldn't be applauding ourselves too much.
> In addition, it seems likely that as more and more people
> get a deeper understanding of Functors, Applicators and
> Monads, we'll find better ways of teaching them.
>  Jón
> --
> Jón Fairbairn                              Jon.Fairbairn at
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