Added combinator functions: `combine{,1,!}()` and `inverse{,*}()`.

Fortune for cl-utils's current commit: Middle blessing − 中吉

flan-utils.lisp

@@ -107,6 +107,8 @@
 (defun true (f)
   (not (null f)))
 
+  ; Functional
+
 (defun nop ()
   "Do nothing"
   nil)
@@ -115,6 +117,42 @@
   "Do nothing"
   t)
 
+					; Combinators
+
+  (defun deatomise (list)
+    "If `list' is a list, pass it through; if it is a non-nil atom, wrap it in a single-element list"
+    (if (and list (atom list))
+	(cons list nil)
+	list))
+
+  (defun deatomise! (list) "Ensure `list' is a list" (cons list nil))
+
+  (defun combine (fa fb &key (pass #'deatomise!))
+    "Returns an applicative lambda that runs (`fb' (`fa' args...)...)
+NOTE: This function *applies* the result of `fa' to `fb', therefore, if the result of `fa' is a list, the elements of said list are applied as the sequential arguments for `fb', if the result is a single element, it is passed as argument 1 only (this includes ``nil''). If you want to pass the result of `fa' to `fb' verbatim through the first argument only, use ``combine1''().
+WARNING: By default, if `fa' returns ``nil'', the ``nil'' is passed as argument 1 to `fb'. Therefore, no function `fa' will ever produce 0 arguments for `fb'; if you wish to override this behaviour and allow a nil return to mean 0 arguments, set `pass' to ``deatomise''() (to still ensure the return of `fa' is contained list; you can use ``combine!''() for behaviour instead too), or the ``identity''() function, if you know `fa' returns a list."
+    (lambda (&rest args) (apply fb (funcall pass (apply fa args)))))
+
+  (defun combine! (fa fb)
+    "Returns an applicative lambda that runs (`fb' [(`fa' args...)...])
+NOTE: This is the same as calling `combine' with `pass' as ``deatomise''()."
+    (combine fa fb :pass #'deatomise))
+
+  (defun combine1 (fa fb)
+    "Returns a lambda that runs (`fb' (`fa' args...))
+NOTE: The difference between this an ``combine''() is that `combine' *applies* the result of `fa' to `fb', whereas `combine1'() simply calls `fb' with the result of `fa'."
+    (lambda (&rest args) (funcall fb (apply fa args))))
+  
+  (defun inverse (func)
+    "Returns a lambda that resolves ¬(`func' args...)"
+    (lambda (&rest n) (not (apply func n))))
+
+  (defun inverse* (&rest functions)
+    "Returns a list of `inverse'()d functions from `functions'"
+    (mapcar #'inverse functions))
+  
+  ;; Mapping
+  
 (defun mapline (input fi &key (read-line #'read-line))
   "Map lines from stream"
   (loop for line = (funcall read-line input nil)