With Parsec, how do I parse zero or more foo1 terminated by foo2 and all separated by dot?

Question

What I am trying to do seems pretty simple, but since I am a parsec Haskell newb, the solution is eluding me.

I have two parsers, let's say foo1 and foo2 where foo1 can parse a intermedate term and foo2 parses an ending term. Terms are separated by a symbol, ".".

Sentences that I need to parse are

• foo2
• foo1.foo2
• foo1.foo1.foo2

and so on.

My original thought was to do

do k <- sepBy foo1 (char'.')
   j <- foo2

but that wouldn't catch the foo2-only case.

Answer 1

You want endBy, not sepBy.

foo = do k <- foo1 `endBy` char '.'
         j <- foo2
         ... 

That will force the separator to be present after each occurrence of foo1.

Of course, endBy is trivially replaceable by many, which may be clearer.

foo = do k <- many $ foo1 <* char '.' 
         j <- foo2
         ...

or, without Control.Applicative:

foo = do k <- many $ do x <- foo1; char '.'; return x
         j <- foo2
         ...