[Haskell-cafe] Optional EOF in Parsec.

[Stephan Friedrichs]

Kannan Goundan wrote:
> I'm writing a parser with Parsec.  In the input language, elements of a sequence
> are separated by commas:
> 
>    [1, 2, 3]
> 
> However, instead of a comma, you can also use an EOL:
> 
>   [1, 2
>   3]
> 
> Anywhere else, EOL is considered ignorable whitespace.  So it's not as simple as
> just making EOL a token and looking for (comma | eol).

Hi Kannan,

let's construct the parser top-down. On the top level, you have opening
and closing characters, '[' and ']'. Parsec has a function for that:

 between (char '[') (char '])

And what's in between? A list of elements separated by something. Parsec
provides a sepBy function for that:

 element `sepBy` separator

which parses a list of elements separated by separator. What's your
separator? Well it's either ',' or a new line and spaces before and
after that:

 mySpaces >> (newline <|> char ',') >> mySpaces -- [1]

Let's combine what we've got:

myListOf :: (Parsec String () a) -> Parsec String () [a]
myListOf elem = between
    (char '[')
    (char ']')
    (elem `sepBy` (mySpaces >> (newline <|> char ',') >> mySpaces))
    where
    mySpaces = many (oneOf (" \t"))

And test it in ghci:

*Main> parseTest (myListOf anyChar) "[a , b, d ,d\np]"
"abddp"

Hope this helps!

 Stephan

PS: The important thing is that there are a lot solutions for tricky
situations (like yours) in Text.Parsec.Combinator (especially the sepBy
and many families). Knowing them can save a lot of work :)

[1] I don't use parsec's spaces function because it also accepts newline
characters.

> 
> I've implemented this functionality in a hand-written parser (basically a hack
> that keeps track of whether the last read token was preceded by an EOL,
> without making EOL itself a token).  Does anybody have ideas about how to
> do this with Parsec?
> 
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