Malcolm writes:
> To be honest, I consider the first of these variants to be an ugly and
> confusing misuse of layout, and the second to be clear (if not the most
> beautiful way to code it). The first seems to be trying to simulate a
> C-style `return', which is in reality not at all like the true monadic
> `return'. I had to read it several times to understand what was
> intended. More than ugly, I think it is dangerous, because of the
> possibility of confusing beginners who are new to both monads and
> layout. Try this variation on the same theme:
>
> mcompare x y =
> do i <- x ==~ y
> if i then return EQ else do i <- x <=~ y
> if i then return LT else do return GT
>
> This code now has a very serious bug! If you use layout properly, as
> with strictly increasing indentation, this confusion is much less
> likely to arise.
I see your point, but this has an error anyway: the second "do" doesn't
end with an expression, but with "i <- x <=~ y", which is a syntax
error.
I often find I *do* want a C-style `return' in code; it's quite natural
in the IO monad, for example, to test some (stateful) flag and only if
it is true, do some stuff. Thus:
possiblyAct :: IO ()
possiblyAct = do f <- checkFlag
if not f then return ()
else do {- stuff -}
If you do this more than once, you find yourself coding hard up against
the right-hand edge of the edit window. Obviously allowing the `else'
to line up with the `if' would help a little here, but not
completely... conceptually, {-stuff-} is in the same path of control as
`f <- checkFlag'.
--KW 8-)
--
: Keith Wansbrough, MSc, BSc(Hons) (Auckland) ------------------------:
: PhD Student, Computer Laboratory, University of Cambridge, England. :
: (and recently of the University of Glasgow, Scotland. [><] ) :
: Native of Antipodean Auckland, New Zealand: 174d47' E, 36d55' S. :
: http://www.cl.cam.ac.uk/users/kw217/ mailto:[EMAIL PROTECTED] :
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