Tabs are particularly useless in a language like Haskell, because we don't do a lot of block-indentation there, but we do a lot of alignment. And your code mixes alignment and indentation.
In a language like C your outermost layers of leading whitespace are always indentation, and then you might have some alignment inside.
But in Haskell you might want to align arguments to a function, but some of the arguments can have blocks inside of them.
Mixing up tabs and spaces is technically possible, but it's too much of a pain in practice to bother.
