[Haskell-cafe] Re: if - then - else layout

25 Sep 2008


      Jules Bean  wrote:
...
do
   if n>5 then
     putStrLn "big"
   else
     putStrLn "small"
this is shorthand for
do { if n > 5 then putStrLn "big" ; else putStrLn "small" }
which is a syntax error. A statement in a do block cannot begin with
the keyword "else".
Haskell' is apparently going to include a hack to permit this case. I 
think that's a poor decision, because including a hack to the layout 
rule makes it harder to understand and explain the layout rule.
There's no need to hack the layout rule, you're even giving pointers to
the solution. Something like this:

if p = do
	(_, c, a) <- get
	put (b, c, a)
	mzero

then c = do
	(b, _, a) <- get
	put (b, c, a)
	mzero

else a = do
	(b, c, _) <- get
	put (b, c, a)
	mzero

end = do
	(b, c, a) <- get
	return if p then a else c

Advantages are obvious: Order doesn't really matter anymore, as in

then "get away"
else "or else"
if i tell you to
end

Furthermore, this scheme supports logical comments, a rare kind of
control structure enabling mindboggingly diverse rapid prototyping
options:

if i knew what i want to do
if i knew how to do it
then i'd have written the next line much earlier
if i wrote this line
then i don't need to remove the other lines
else where in other languages i'd have to do that
end

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[Haskell-cafe] Re: if - then - else layout

Achim Schneider