Haskell -- parser combinators keywords

I am working on building a parser in Haskell using parser combinators. I have an issue with parsing keywords such as "while", "true", "if" etc

So the issue I am facing is that after a keyword there is a requirement that there is a separator or whitespace, for example in the statement if cond then stat1 else stat2 fi;x = 1 with this statement all keywords have either a space in front of them or a semi colon. However in different situations there can be different separators.

Currently I have implemented it as follows:

keyword :: String -> Parser String
keyword k = do
  kword <- leadingWS (string k)
  check (== ';') <|> check isSpace <|> check (== ',') <|> check (== ']')
  junk
  return word

however the problem with this keyword parser is that it will allow programs which have statements like if; cond then stat1 else stat2 fi

We tried passing in a (Char -> Bool) to keyword, which would then be passed to check. But this wouldn’t work because where we parse the keyword we don’t know what kind of separator is allowed.

I was wondering if I could have some help with this issue?

Solution

Don't try to handle the separators in keyword but you need to ensure that keyword "if" will not be confused with an identifier "iffy" (see comment by sepp2k).

keyword :: String -> Parser String
keyword k = leadingWS $ try (do string k
                                notFollowedBy alphanum)

Handling separators for statements would go like this:

statements = statement `sepBy` semi
statement  = ifStatement <|> assignmentStatement <|> ...