Wolfgang Thaller writes (to the Haskell mailing list):

...

IMHO, there should only be warnings about tabs when their size makes a difference to the meaning of the program,

I agree and would suggest an even more stringent test to warn against

...

a = let x = 1 y = 2 -- OK in ...

because the (visual) interpretation depends on the font you use (fixed width vs. proportional).

The Revised Haskell 98 Report contains this paragraph (sec. 9.3): For the purposes of the layout rule, Unicode characters in a source program are considered to be of the same, fixed, width as an ASCII character. However, to avoid visual confusion, programmers should avoid writing programs in which the meaning of implicit layout depends on the width of non-space characters. As for the width of the tab character: tab stops are every 8 columns. Period. The Haskell report says so :-P Cheers, Simon