[Haskell-beginners] Operator orders affect parse result

[Francesco Ariis]

On Mon, Apr 13, 2015 at 02:56:47PM +0800, m00nlight wrote:
>  Dear Haskellers,
> 
> I have tried the "while language parser" tutorial on haskellwiki, but I
> encounter an problem I can not understand here,
> The following is the code:
> 
> http://pastebin.com/saC9vwCn
> 
> 
> So it seems that the order of the operator will affect the parse result.
> What's the problem with my code?  How can I make my program to produce
> consistent parse result.

Checking the documentation for OperatorTable [1] (the first argument to
buildExpressionParser), it seems it is working as expected:

    An OperatorTable s u m a is a list of Operator s u m a lists. The list
    is ordered in descending precedence. All operators in one list have
    the same precedence (but may have a different associativity).

I think you can take advantage of the fact that OperatorTable is /a list
of lists/, hence

    aOperators = [ [Prefix (reservedOp "-" >> return (Neg ))]
                 , [Infix (reservedOp "/" >> return (ABinary Divide )) AssocLeft,
                    Infix (reservedOp "*" >> return (ABinary Multiply )) AssocLeft]
                 , [Infix (reservedOp "+" >> return (ABinary Add )) AssocLeft]
                 , [Infix (reservedOp "-" >> return (ABinary Subtract )) AssocLeft]
                 ]

should do the trick (try swapping the elements in the second list.
Will this do?


[1] http://hackage.haskell.org/package/parsec-3.0.0/docs/Text-Parsec-Expr.html#t:OperatorTable