Question

I’m attempting to write a parser in Haskell using Parsec. Currently I have a program that can parse

test x [1,2,3] end

The code that does this is given as follows

testParser = do { 
  reserved "test"; 
  v <- identifier; 
  symbol "["; 
  l <- sepBy natural commaSep;
  symbol "]";
  p <- pParser;
  return $ Test v (List l) p
 } <?> "end"

where commaSep is defined as

commaSep        = skipMany1 (space <|> char ',')

Now is there some way for me to parse a similar statement, specifically:

test x [1...3] end

Being new to Haskell, and Parsec for that matter, I’m sure there’s some nice concise way of doing this that I’m just not aware of. Any help would be appreciated.

Thanks again.

Answer 1

I’ll be using some functions from Control.Applicative like (*>). These functions are useful if you want to avoid the monadic interface of Parsec and prefer the applicative interface, because the parsers become easier to read that way in my opinion.

If you aren’t familiar with the basic applicative functions, leave a comment and I’ll explain them. You can look them up on Hoogle if you are unsure.

---

As I’ve understood your problem, you want a parser for some data structure like this:

data Test = Test String Numbers
data Numbers = List [Int] | Range Int Int

A parser that can parse such a data structure would look like this (I’ve not compiled the code, but it should work):

-- parses "test <identifier> [<numbers>] end"
testParser :: Parser Test
testParser =
  Test <$> reserved "test" *> identifier
       <*> symbol "[" *> numbersParser <* symbol "]"
       <*  reserved "end"
       <?> "test"

numbersParser :: Parser Numbers
numbersParser = try listParser <|> rangeParser

-- parses "<natural>, <natural>, <natural>" etc
listParser :: Parser Numbers
listParser =
  List <$> sepBy natural (symbol ",")
       <?> "list"

-- parses "<natural> ... <natural>"
rangeParser :: Parser Numbers
rangeParser =
  Range <$> natural <* symbol "..."
        <*> natural
        <?> "range"