[Haskell-cafe] Records (was Re: [Haskell] Improvements to GHC)

Cale Gibbard cgibbard at gmail.com
Thu Nov 17 17:44:05 EST 2005

On 17/11/05, Sebastian Sylvan <sebastian.sylvan at gmail.com> wrote:
> On 11/17/05, Cale Gibbard <cgibbard at gmail.com> wrote:
> > On 17/11/05, Sebastian Sylvan <sebastian.sylvan at gmail.com> wrote:
> > > On 11/17/05, Greg Woodhouse <gregory.woodhouse at sbcglobal.net> wrote:
> > > > Isn't there a potential for confusion with function composition (f . g)?
> > > >
> > > > That being said, I like this idea (I just need to think it through a bit).
> > > >
> > >
> > > I've been wanting this for ages. It's SO much better than the current
> > > horribly broken records we have.
> > > There could be confusion with function composition, but there's no
> > > ambiguity (compositon have spaces around the dot, while record
> > > accessors do not).
> > > Personally I think that the dot is way to good of a symbol to be
> > > "wasted" on function composition. I mean, how often do you really use
> > > function composition in a way which doesn't obfuscate your code? I use
> > > ($) way more often than (.). Some people do use it more often than I
> > > do, but I find that in most cases except simple "pipelined" functions
> > > it only makes the code harder to read.
> > > I'd rather function composition was left out of the prelude
> > > alltogether (or defined as (#) or something).
> > >
> > > Anyway. The current records system is a wart.
> > >
> >
> > Actually, I didn't mention this in the other post, but why not the
> > other way around? Make record selection (#) or (!) (though the latter
> > gets in the way of array access), and leave (.) for function
> > composition. Personally, I'd like something which looked like an arrow
> > for record selection, but most of the good 2-character ones are
> > unavailable. (~>) is a bit hard to type and looks wrong in some fonts.
> > There's a "triangle" which is not taken, and isn't so hard to type
> > (|>).
> >
> > I never really understood the attachment to (.) for record selection.
> > There's no reason that we have to make things look like Java and C.
> This is going to be highly fuzzy and completely subjective. Here it goes.
> I find that for selections (records, or qualified modules etc.) I want
> the operator to be small and so that the important "word groups"
> become the module or the record.
> When I read the following two variants
> myPoint#x
> myPoint.x

I think both of those look crowded -- smashing operator punctuation up
against symbols basically never looks good to me. The right amount of
spacing isn't generally available without proper typesetting, but a
full space is a lot closer than no space at all.

Why not "myPoint # x" and "myPoint . x"?

> I definatly prefer the latter. In the first one the operator is so
> large that it makes "myPoint" and "x" blend together as you read it
> (step away from the monitor and squint and you'll see what I mean),
> whereas in the second example the operator is small and makes the two
> operands naturally separate slightly when reading it, which makes it
> easier to tell which identifier is accessed. Also, it's certainly not
> a BAD thing if Haskell uses the same operators as other languages.
> With function composition, though, the operator is just as important
> to identify when reading as the operands are. So I don't think a "big"
> operator is a problem there - likewise I have no problems with ($)
> being "large".
> How about (¤)? It looks like a ring to me, I'm not sure where that's
> located on a EN keyboard, but it's not terribly inconvenient on my SE
> keyboard. f ¤ g looks better than "f . g" for function composition, if
> you ask me.
That symbol actually does look better, but isn't on any English
keyboards to the best of my knowledge. I can get it in my setup with
<compose-key> o x, but not many people have a compose key assigned.
Also, this may just be a bug, but currently, ghc gives a lexical error
if I try to use that symbol anywhere, probably just since it's not an
ASCII character.
> That's my subjective view on why the dot-operator is so darn nice, anyway.
> Oh and to answer to your other post. I realise that function
> composition is a fundamental operation, but it's so fundamental that
> it's quite useless for most real-world cases unless your willing to
> seriously ubfuscate your code.

I disagree, there are plenty of cases where it's just what you want,
and saves you from introducing a lambda term for nothing. This occurs
very often in parameters to higher order functions. A simple example
would be something like "filter (not . null)", or "any ((`elem`
consumers) . schVertex)". More sophisticated examples come up all the
time, and often the functions being composed have some parameters
applied to them. I disagree that it's just for obfuscation. Using
function composition puts emphasis on the manipulation of functions
rather than on the manipulation of the elements those functions act
on, and quite often in a functional language that's just what you

> IMO it really only works well for simple "chains" like "foo . bar .
> oof . rab" but as soon as you start working with functions that take
> more parameters it starts looking very unreadable and you'd be better
> off to just use $ or write out paranthesis and apply arguments
> explicitly, or better yet, introduce some temporary descriptive
> variables in a let or where clause.
> It's a matter of personal preference, but I certainly haven't found it
> used enough to warrant giving it perhaps the best symbol on the
> keyboard.

The best symbol on the keyboard is almost certainly space, which
currently goes to function application. :) Personally, I really like
the current syntax, which just makes the labels into projection
functions when that's appropriate.

 - Cale

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