Beginner: error when using multiple where stmts in hugs98

Guy Lapalme lapalme@IRO.UMontreal.CA
Fri, 01 Dec 2000 14:30:59 -0500

Mark and Win-Jan

> | I've been trying some examples in functional programming. Most things
> | work fine, but I have trouble with expressions with 'where' clauses
> | that define more then one local definition.
> | ...
> | For example:
> | ...
> | mydiff f = f'
> |         where f' x = ( f (x+h) - f x) / h
> |                     h = 0.0001
> The second line in the where clause (your attempt to define local
> variable h) will be treated as if it were part of the first definition
> (for the variable f').  Why?  Because that's what the layout/indentation
> in your code specifies.  In fact, to a Haskell compiler, your input
> looks a lot like the following, clearly incorrect definition:
> mydiff f = f'
> where f' x = ( f (x+h) - f x) / h h = 0.0001

this explanation is very good but the real confusion comes from the fact
that probably there were some TAB characters in the original program. In the
text editor used for typing the program the tabs stops were 4 spaces apart
as they are usually nowadays because otherwise programs become too wide...

But the Haskell report (section B.3) specifies that the TAB characters are 8
characters apart. This is a small point indeed compared to existential
polymorphic typing problems, but as author the Haskell Emacs mode, I have
been asked quite a few times about the explanation of this kind of behavior.

Concrete suggestions about this any one?

Guy Lapalme
PS: of course, one can specify that tabs insert spaces but then one must be
cautious to untabify existing programs.