[Haskell-cafe] Parsec question (new user): unexpected end of input

29 Sep 2010

      I am a new Parsec user, and having some trouble with a relatively
simple parser.

The grammar I want to parse contains tags (not html) marked by
angle brackets (e.g., "<some tag>"), with arbitrary text (no angle
brackets allowed) optionally in between tags.

Tags may not nest, but the input must begin and end with a tag.

Whitespace may occur anywhere (beginning/end of input,
inside/between tags, etc.), and is optional.

I think my problem may be a lack of using "try", but I'm not sure
where.

At runtime I get:

Error parsing file: "...\sampleTaggedContent.txt" (line 4, column 1):
unexpected end of input
expecting "<"

The input was:

<tag1>stuff<tag 2>
more stuff < tag 3 > even more
<lastTag>

The code is below. (I'm using Parsec-2.1.0.1.) I don't really want
to return anything meaningful yet; just parse okay.

Any advice about the error (or how to simplify or improve the code)
would be appreciated.

Thanks much,
-- Peter
...
-- Parsers:
taggedContent = do
   optionalWhiteSpace
   aTag
   many tagOrContent
   aTag
   eof
   return "Parse complete."
tagOrContent = aTag <|> someContent <?> "tagOrContent"
aTag = do
   tagBegin
   xs <- many (noneOf [tagEndChar])
   tagEnd
   optionalWhiteSpace
   return ()
someContent = do
   manyTill anyChar tagBegin
   return ()
optionalWhiteSpace = spaces   -- i.e., any of " \v\f\t\r\n"
tagBegin = char tagBeginChar
tagEnd = char tagEndChar
-- Etc:
tagBeginChar = '<'
tagEndChar = '>'
--------

[Haskell-cafe] Parsec question (new user): unexpected end of input

Peter Schmitz