[Haskell-cafe] Am I using Parsec correctly?

[Mario Blažević]

On 03/03/15 05:00 AM, Cody Goodman wrote:
> -- trying to parse the text below using Parsec:
>
> -- ACT I.
>
> -- SCENE I. An open place.
> -- [An open place. Thunder and lightning. Enter three Witches.]
>
> --   FIRST WITCH.
> --     When shall we three meet again
> --     In thunder, lightning, or in rain?
>
> -- Here's the code I have
>
> import Text.Parsec
> import Control.Applicative
> import Data.Traversable (sequenceA)
>
> section s = sequenceA [string s, string " ", many1 letter] `endBy` 
> char '.'
>
> act = section "ACT"
> scene = section "SCENE"
>
> main = do
>   parseTest act "ACT I.\n"
>   parseTest scene "SCENE I."
>
> -- this returns:
> -- λ> main
> -- [["ACT"," ","I"]]
> -- [["SCENE"," ","I"]]
>
> -- Am I using Parsec correctly so far?

     The act, scene, and section functions are just fine. The main 
function is probably not, unless you really intend your act and scene to 
be two different inputs. The parseTest function should generally be used 
only once, and it should be applied to the entire parser. So what you 
want is more like

> parser = do act
>             string "\n"
>             scene
>
> main = parseTest parser "ACT I.\nSCENE I."

     Note that the result of the act function is being discarded in this 
example. You'll probably want to preserve it somehow, but unless you 
just want to print it you'll need to decide what kind of result you want 
to construct. It's generally a good idea to do that before setting out 
to write the parser itself.