parsing - How exactly is readsPrec supposed to work? -

i newbie haskell. making read instance of custom type, decimalfp:

data integral => decimalfp = decimalfp {infprec :: bool, digits :: a, exp :: a} 

which used decimal floating-point type specific significant figures.
i'm unsure how readsproc works, these issues:

1. why return type list?

when this:

prelude> readsprec 0 "21345 1223 -5" :: [(int, string)] 

i expected this:

[(21345," 1223 -5"),(1223," -5"),(-5,"")] 

but actual result just:

[(21345," 1223 -5")] 

so when readsprec return multiple elements?

2. return type seems biased?

this works expected:

prelude> readsprec 0 "245e-3" :: [(float, string)] 

it outputs [(0.245,"")]. when this:

prelude> readsprec 0 "245e-3" :: [(int, string)] 

it outputs []. expected [(245,"e-3")]. why happen?

3. first argument do?

i know has operator precedence, exactly?

edit: besides, here completed read instance of decimalfp:

instance (integral a) => read (decimalfp a)     readsprec p cs =         let (bigdigits, rem1) = parsesigned cs         let (_, rem1') = parsediscard "." rem1         let (smalldigits, rem2) = parseunsigned rem1'         let (succ, rem2') = parsediscard "ee" rem2         let (expdigits, rem3) = if succ parsesigned rem2' else ("0", rem2')         let digitshift = frominteger $ tointeger $ length smalldigits         digit <- map (frominteger . fst) (readsprec p $ bigdigits ++ smalldigits :: [(integer, string)])         exp <- map (frominteger . fst) (readsprec p expdigits :: [(integer, string)])         return (decimalfp false digit $ exp - digitshift, rem3)             parseunsigned, parsesigned :: string -> (string, string)             parseunsigned = span (`elem` "0123456789")             parsesigned str@(c:cs) = if c `elem` "-0123456789"                 let (digits, rem) = parseunsigned cs in (c:digits, rem)                 else ("", str)             parsesigned _ = ("", "")             parsediscard :: string -> string -> (bool, string)             parsediscard s1 str@(c:cs) = let succ = c `elem` s1 in (succ, if succ cs else str)             parsediscard _ _ = (false, "")