MAP: infer return type of mapping function

[Michał "phoe" Herda]

As per https://markkarpov.com/post/lisp-and-haskell.html :

There is a function mentioned in the above blog post, the full body of which is:

(defun add-text-padding (str &key padding newline)
  "Add padding to text STR. Every line except for the first one, will be
prefixed with PADDING spaces. If NEWLINE is non-NIL, newline character will
be prepended to the text making it start on the next line with padding
applied to every single line."
  (let ((str (if newline
                 (concatenate 'string (string #\Newline) str)
                 str)))
    (with-output-to-string (s)
      (map 'string
           (lambda (x)
             (princ x s)
             (when (char= x #\Newline)
               (dotimes (i padding)
                 (princ #\Space s))))
           str))))

The troublesome part is the (MAP 'STRING (LAMBDA (X) ...) STR) call. The anonymous function ends with a WHEN/DOTIMES call, which causes it to always return NIL. MAP 'STRING, in turn, requires that its function returns characters.

This information can be utilized by SBCL to emit a compile-time warning about mismatched types: the type exected by MAP and actual return type of the function.

[Stas Boukarev (stassats)]

In da13f47cceaacafa6f4f60917032c2cdb22add5e.

[Michał "phoe" Herda (phoe-krk)]

That was fast! Thank you!