Tokenizer identifier in Haskell -

i'm writing small program identify each input tokens operator/parenthesis/int.

however, encountered problem stating

not in scope: data constructor `integer' 

here's have far (data.char defines isdigit, nothing else)

import data.char (isdigit) data token = tplus | ttimes | tparenleft | tparenright | tnumber integer | terror     deriving (show, eq)   tokenize :: string -> [token] tokenize [] = [] tokenize (c:cs)     | c == '+' = tplus : tokenize cs     | c == '*' = ttimes : tokenize cs     | c == '(' = tparenleft : tokenize cs     | c == ')' = tparenright : tokenize cs     | isdigit c = tnumber integer (read c) : tokenize cs     | otherwise = terror : tokenize cs 

some example expected output:

*main> tokenize "( 1 + 2 )" 

should give

[tparenleft,tnumber 1,tplus,tnumber 2,tparenright] 

and

*main> tokenize "abc" 

should expect terror, i'm getting

[terror,terror,terror] 

i'd appreciate if shed light on these 2 issues.

for not in scope: data constructor 'integer' part, problem have integer in line

isdigit c = tnumber integer (read c) : tokenize cs 

which should be

isdigit c = tnumber (read [c]) : tokenize cs 

the [c] part needed because read has type read :: read => string -> a, , c char, [c] string containing char c.

---

tokenize "abc" returning [terror, terror, terror] because of error treatment policy:

| otherwise = terror : tokenize cs 

this leads to:

tokenize "abc" -- c = 'a', cs = "bc" terror : tokenize "bc" terror : (terror : tokenize "c") terror : terror : terror : [] [terror, terror, terror] 

if want group of errors in single terror, should drop incorrect input

| otherwise = terror : (dropwhile (\o -> o == terror) (tokenize cs))