Indentation of If-Then-Else

[Henrik Nilsson]

Dear All,

Henning Thielmann wrote:

 > I object strongly to the proposal
 >  http://hackage.haskell.org/trac/haskell-prime/wiki/DoAndIfThenElse
 > because it solves problems with syntactic sugar with even more sugar,
 > where no sugar is needed at all.
 >
 > In order to solve the trouble I propose enhancements
 > to teachers, compilers and standard libraries:
 > 1. 'if' syntax should be teached as
 >      if a
 >        then b
 >        else c
 >    This indentation makes pretty clear,
 >    what is condition, and what is the result in both cases.

People obviously have different opinions here, and I'd say
there are perfectly reasonable arguments for a number of
different styles of indentation. As the discussion following
Henning's mail has demonstrated.

The background to the proposal was that Haskell 98 prevents
an arguably reasonable style of indentation, and that this
has turned out to be a problem in practice: i.e. it tends to
trip up a lot of unsuspecting people, in particular beginners.

Yes, one can argue that they should learn "the right way",
but this is really a very minor detail that many think would
be best if people didn't have to worry about in the first place.

The proposal is actually very lightweight (just allow an
optional ";" in the appropriate place), and thus it is not
even a question about new "syntactic sugar". At least not
according to my understanding of the term. Also, it
does not complicate the (already complicated) layout rules
further, which is quite important.

If I recall correctly, the proposal was implemented in GHC
(and JHC?) shortly after it had been put forward, with
very little effort indeed, and has not caused any ill
side effects that I'm aware of.

 > 3. If a compiler suspects that a parsing problem is related to 'do'
 > and 'if'
 >    it should suggest indentation
 >      if a
 >        then b
 >        else c
 >    or even better, the usage of if- or (?)-function.

As a matter of principle, I don't think a language standard should say
much if anything about how a conforming compiler or interpreter should
report errors.

We can argue the pros and cons of the existing "if" syntax, but I
don't think anyone is seriously proposing that it be dropped from
Haskell' (that would break an awful lot of code for very little
gain). Having additional functions that accomplish the same thing
as "if" does not seem all that appealing to me at least.

 > So, please add this to the 'Cons' list.

Henning, I'm afraid I don't quite understand what the conses are?
Could you clarify the concrete ill effects of the proposal, please?

Best,

/Henrik

-- 
Henrik Nilsson
School of Computer Science and Information Technology
The University of Nottingham
nhn at cs.nott.ac.uk

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