I think it has something to do with which unicode symbols are punctuation. 

Check out this StackOverflow answer:

http://stackoverflow.com/questions/10548170/what-characters-are-permitted-for-haskell-operators

On Wed, Apr 13, 2016 at 12:33 PM, Silent Leaf <silent.leaf0@gmail.com> wrote:

Hi!

say I wanna use "—" as new infix operator: it's the big dash used a bit like parenthesis, especially at the end of sentences —like this.

in ghci directly:
Prelude> let (—) a b = a + b

No problem, is accepted and usable. Same in files.

Now I try using (»), a French (amongst others) punctuation sign, typically replaces the quote-ends, « like this ».
Doesn't work:
<interactive>:2:6: lexical error at character '\187'

I thought Haskell was Unicode-friendly? Why some symbols but not others? :'(

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