Emptylist issues when using parsec

Question

Here is some of the code which is causing some very strange errors depending on what is parsed after the token emptyList in the target file.

data Express = Seq [Express]
              | ID String
              | Num Integer
              | BoolConst Bool
              | EmptyList String

emptyList :: Parser Express
emptyList = fmap EmptyList (string "[]") 

The issue is:

When the string "[]" is followed by anything (including whitespace) other than a digit or the end of the file it causes the following error

(line 1, column 6):
unexpected end of input   OR    unexpected "d"
expecting digit
*** Exception: user error (parse error) 

However when parsed with a digit following the [] it reports no error.

e.g. "[] "  error 
     "[] a" error
     "[] 1" successful

Also should the Num Integer be removed from the data definition and elsewhere in the file

number :: Parser Express
number = fmap Num integer

The file will parse fine without anything after the "[]" however parsing will return only up until the first occurrence of "[]" and nothing after it

Thanks in advance.

Answer 1

The issue was coming from the way that the expressions were being delimited, whiteSpace was not being accounted for correctly when dealing with multiple expressions. initially:

seqOfExpr8 =        
    do list <- (sepBy1 expr8 (string "" ) ) 
       return $ if length list == 1 then head list else Seq list     

fixed:

seqOfExpr8 =        
    do list <- (sepBy1 expr8 whiteSpace ) 
       return $ if length list == 1 then head list else Seq list     

where whiteSpace is a lexeme parser from genTokenParser Type.