Re: [Haskell-cafe] Associative prefix operators in Parsec
Am 08.03.2012 17:16, schrieb Troels Henriksen:
Christian Maederchristian.mae...@dfki.de writes:
The simplest solution is to parse the prefixes yourself and do not put
it into the table.
(Doing the infixes and | by hand is no big deal, too, and
possibly easier then figuring out the capabilities of
buildExpressionParser)
Is there another solution? My post was a simplified example to showcase
the problem; in general I would prefer to use a function to build the
expression parser. I could just write my own that does not have this
problem, and in fact, I already have, I just wanted to know whether
Parsec could be wrangled into shape.
Yes, it certainly could do better. The code for prefix and postfix
currently looks like:
termP = do{ pre - prefixP
; x- term
; post - postfixP
; return (post (pre x))
}
This supports (only) one prefix or postfix (or both), where the prefix
binds stronger than the postfix (although, they have equal precedence).
Problem 1: - - 5 is not supported
Another problem are prefix or postfix operators that bind weaker than
infixes, like infix ^ and prefix -.
Problem 2: 1 ^ -2 is rejected, although no other parsing is possible.
(The same would apply to a weakly binding postfix operator following the
left argument: 4+ ^ 5. This would even need some look ahead to find
out, if + is not an infix and ^ a prefix operator)
(Haskell features these problems, too.)
Maybe the special case of repeated prefixes could be solved by putting
in the prefix entry twice into the table for the
currentbuildExpressionParser, but considering possibly equal symbols for
prefix, postfix or infix symbols seems quite difficult, although only an
ambiguity needs to be reported.
Cheers C.
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Re: [Haskell-cafe] Associative prefix operators in Parsec
The simplest solution is to parse the prefixes yourself and do not put
it into the table.
(Doing the infixes and | by hand is no big deal, too, and possibly
easier then figuring out the capabilities of buildExpressionParser)
Cheers C.
Am 07.03.2012 13:08, schrieb Troels Henriksen:
Consider a simple language of logical expressions:
import Control.Applicative
import Text.Parsec hiding ((|), many)
import Text.Parsec.String
import Text.Parsec.Expr
data Expr = Truth
| Falsity
| And Expr Expr
| Or Expr Expr
| Not Expr
deriving (Show, Eq)
I define a simple expression parser using Parsec:
expr :: Parser Expr
expr= buildExpressionParser table (lexeme term)
? expression
term :: Parser Expr
term= between (lexeme (char '(')) (lexeme (char ')')) expr
| bool
? simple expression
bool :: Parser Expr
bool = lexeme (string true * pure Truth)
| lexeme (string false * pure Falsity)
lexeme :: Parser a - Parser a
lexeme p = p* spaces
table = [ [prefix ! Not ]
, [binary And AssocLeft ]
, [binary | Or AssocLeft ]
]
binary name fun assoc = Infix (do{ lexeme (string name); return fun }) assoc
prefix name fun = Prefix (do{ lexeme (string name); return fun })
Now this doesn't work:
test1 = parseTest expr !!true
But this does:
test2 = parseTest expr !(!true)
I have studied the code for buildExpressionParser, and I know why this
happens (prefix operators are treated as nonassociative), but it seems
like one would often want right-associative prefix operators (so test1
would work). Is there a common workaround or solution for this problem?
I assume the nonassociativity in Parsec is by design and not a bug.
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Re: [Haskell-cafe] Associative prefix operators in Parsec
Christian Maeder christian.mae...@dfki.de writes: The simplest solution is to parse the prefixes yourself and do not put it into the table. (Doing the infixes and | by hand is no big deal, too, and possibly easier then figuring out the capabilities of buildExpressionParser) Is there another solution? My post was a simplified example to showcase the problem; in general I would prefer to use a function to build the expression parser. I could just write my own that does not have this problem, and in fact, I already have, I just wanted to know whether Parsec could be wrangled into shape. -- \ Troels /\ Henriksen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Associative prefix operators in Parsec
Consider a simple language of logical expressions:
import Control.Applicative
import Text.Parsec hiding ((|), many)
import Text.Parsec.String
import Text.Parsec.Expr
data Expr = Truth
| Falsity
| And Expr Expr
| Or Expr Expr
| Not Expr
deriving (Show, Eq)
I define a simple expression parser using Parsec:
expr :: Parser Expr
expr= buildExpressionParser table (lexeme term)
? expression
term :: Parser Expr
term= between (lexeme (char '(')) (lexeme (char ')')) expr
| bool
? simple expression
bool :: Parser Expr
bool = lexeme (string true * pure Truth)
| lexeme (string false * pure Falsity)
lexeme :: Parser a - Parser a
lexeme p = p * spaces
table = [ [prefix ! Not ]
, [binary And AssocLeft ]
, [binary | Or AssocLeft ]
]
binary name fun assoc = Infix (do{ lexeme (string name); return fun }) assoc
prefix name fun = Prefix (do{ lexeme (string name); return fun })
Now this doesn't work:
test1 = parseTest expr !!true
But this does:
test2 = parseTest expr !(!true)
I have studied the code for buildExpressionParser, and I know why this
happens (prefix operators are treated as nonassociative), but it seems
like one would often want right-associative prefix operators (so test1
would work). Is there a common workaround or solution for this problem?
I assume the nonassociativity in Parsec is by design and not a bug.
--
\ Troels
/\ Henriksen
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