[Haskell-beginners] Operator orders affect parse result
m00nlight
dot_wangyushi at yeah.net
Mon Apr 13 08:40:42 UTC 2015
Hi Norbert, Francesco,
Thanks. Just an stupid mistake when I change the code. It is actually correct.
Thanks for all your help.
--m00nlight
在2015年04月13 16时29分, "Norbert Melzer"<timmelzer at gmail.com>写道:
According to the given code, that result seems to be fine... Apply / to a and be and that result is applied to * together with c. Only odd thing is that you named the / Multiply and the * Divide.
Am 13.04.2015 09:50 schrieb "m00nlight" <dot_wangyushi at yeah.net>:
Hi Francesco,
Thanks for your reply. But after I change the aOperators like the following, it still produce non-correct parse result.
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]
]
parse result:
*Main> parseString "res := a / b * c"
Assign "res" (ABinary Divide (ABinary Multiply (Var "a") (Var "b")) (Var "c"))
*Main>
--m00nlight
在2015年04月13 15时21分, "Francesco Ariis"<fa-ml at ariis.it>写道:
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
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