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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