[Haskell-cafe] Re: Proving my point

Achim Schneider barsoap at web.de
Fri May 16 15:33:47 EDT 2008

Andrew Coppin <andrewcoppin at btinternet.com> wrote:

> Wait... "unexpected end of input; expecting [...] end of input [...]"
> That's just *wrong*...! ;-)
> But don't despaire - show us your parser and what it's supposed to 
> parse, and I'm sure somebody [maybe even me] will be able to tell you 
> what's up.

This is what I came up with while simplifying the parser:

import Text.Parsec

identifier = do
    s <- many1 letter
    return s

whiteSpace = do 
    eof <|> ((many $ choice [ char ' ', newline ]) >> return ())

main = do 
    let syn = runParser (do
        char '\\'
        many1 identifier
        char ':'
        ) () "" "\\a b"
    print syn

Admittedly, this is a quite degenerate case crouching in at least 10
corners simultaneously. Anyway, I get

% ./test
Left (line 1, column 5):
unexpected end of input
expecting end of input, letter or ":"

and if I change it to

whiteSpace = do 
    (many eof >> return ()) 
    <|> ((many $ choice [ char ' ', newline ]) >> return ())

Left (line 1, column 3):
unexpected " "
expecting letter, end of input or ":"

Please, please don't ask me for the rationale of using eof like this,
you would get the same answer as if you'd ask me why I cast a stone into
the sea.

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