Separate the maximal decimal prefix from the unconsumed suffix, rejecting an empty prefix:
import Data.Char (isDigit, digitToInt)

type Parse a = String -> Maybe (a,String)

parseInt :: Parse Int
parseInt input =
  case span isDigit input of
    ([],_)  -> Nothing
    (ds,ys) -> Just (foldl (\n d -> 10*n + digitToInt d) 0 ds, ys)
The left fold uses the positional notation recurrence . This parses unsigned decimal operands as illustrated, leaving the first nondigit and everything after it untouched. As the required result type is Int, correct mathematical values require the parsed integer to fit that bounded type; arbitrary-length exact values would instead need Integer or an explicit integer overflow check.

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