Use of tab characters in indentation-sensitive code

[Wolfgang Thaller]

Graham Klyne wrote:

> I think that compilers should issue a warning when indentation that
> determines the scope of a construct is found to contain tab characters.

I'd say, when it "is found to contain a mixture of tab and space 
characters".
I have successfully written a lot of Haskell code that uses tabs 
*exclusively* - in that case, the meaning of the program *doesn't* 
depend on how the tab characters are interpreted.
IMHO, there should only be warnings about tabs when their size makes a 
difference to the meaning of the program, as shown in the examples 
below:

let
<spaces>x = 1
<TAB--->y = 1	-- warning

let
<TAB--->x = 1 -- OK
<TAB--->y = 2 -- OK
<spaces>z = 3 -- warning

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

b = let x = 1
<TAB--->y = 2 -- warning
     in ...

There are many editors that automatically mix tabs and spaces in 
indentation (and I don't like that - what's it good for?), but some 
people will certainly want to continue to use them, so I'm not sure if 
adding warnings like these would be acceptable to them.

Cheers,

Wolfgang