---> being mislexed at the start of a line?

[Donald Bruce Stewart]

igloo:
> 
> Looks like ---> at the start of a line is being mislexed as >:
> 
> $ cat Q.hs 
> 
> module Q where
> a ---> b = a + a
> foo = 3
>  ---> 4
>  ---> 5
> $ ghc -c Q.hs -ddump-parsed
> 
> ==================== Parser ====================
> module Q where
> ---> a b = a + a
> foo = (3 > 4) > 5
> 
> 
> 
> Q.hs:4:6:
>     precedence parsing error
>         cannot mix `(>)' [infix 4] and `(>)' [infix 4] in the same infix expression

Now, this should be lexed as a varsym, and looking through
compiler/parser/Lexer.x, you see the single line comment parsing code:

line 111:
        "--"\-* ([^$symbol] .*)?        ;

The associated comment:
        The regex says: "munch all the characters after the dashes, as
        long as the first one is not a symbol".

But I read the whole regex as:
        "Munch dashes. Also munch all characters after the dashes, as
        long as the first one is not a symbol".

There's the bug, I think. So none of these will work either: --> --<
-->=  etc. So the regex might need to explicitly match varsyms, and rule
them out first. A quick hack:

        "--"\-* $symbol+        { goto varsym code }
        "--"\-* .*              ;

-- Don