Stephane Bortzmeyer wrote:
I'm trying to use Parsec for a language which have identifiers where
the '-' character is allowed only inside identifiers, not at the start
or the end.
ParsecToken has identStart to tell that the '-' is not allowed at the
start but I find no equivalent identEnd?
I have not used ParsecToken
I tried also to express the same rule with ordinary combinators,
without ParsecToken but this fails:
identifier = do
start <- letter
rest <- many (alphaNum <|> char '-')
end <- letter
return ([start] ++ rest ++ [end])
<?> "characters authorized for identifiers"
because the parser created by "many" is greedy: it consumes
everything, including the final letter.
Any idea?
The hard thing about using Parsec is to know how to combine <|> with 'try'.
Fixing this may be as simple as
> identifier = try $ do
> start <- letter
> rest <- many (alphaNum <|> char '-')
> end <- letter
> return ([start] ++ rest ++ [end])
> <?> "characters authorized for identifiers"
Alternatively, if the first character being a letter commits you to an
identifier or a syntax error, then you could move the try after the first letter
has been read and committed to:
identifier = do
start <- letter
try $ do
rest <- many (alphaNum <|> char '-')
end <- letter
return (start:(rest ++ [end]))
<?> "characters authorized for identifiers"
(Both untested)
And can the last letter be an alphaNum instead of only a letter?
You can also make the test more explicit:
> import Data.Char; import Control.Monad;
>
identifier = try $ do
start <- letter
rest <- many (satisfy (\c -> alphaNum c || (c=='-')))
when (not (null rest) && '-' == last rest) (unexpected "Identifier cannot end in
-")
return (start:rest)
or
identifier = do
start <- letter <?> "Identifiers must start with a letter"
try $ do
rest <- many (satisfy (\c -> alphaNum c || (c=='-'))) <?> "valid identifier
character"
when (not (null rest) && '-' == last rest) (unexpected "identifier cannot end in
-")
return (start:rest)
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