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;;; hofs.lisp -- higher-order functions
(in-package :lambda-riffs)
(defun lambda-if (pred then &optional (else (constantly nil)))
"Returns a function that applies PRED and conditionally executes
THEN when PRED returned non-nil or otherwise executes ELSE. Each of
PRED, THEN, and ELSE are assumed to accept the same arguments."
(lambda (&rest args)
(if (apply pred args)
(apply then args)
(apply else args))))
(defun lambda-cond (&rest pairs)
"PAIRS is a list: PRED1 F1 PRED2 F2 ...
Returns a function that applies FN to the argumetns for the first N
such that PREDN returns non-nil. Each of the PREDN and FN are
assumed to accept the same arguments."
(lambda (&rest args)
(loop for (pred fn . more) on pairs by #'cddr
when (apply pred args)
return (apply fn args))))
(defun mapped (fn1 &rest fns)
"Return a function the arguments of which are applied to (FN1 . FNS),
returning a list of return values."
(lambda (&rest args)
(loop :for fn :in (cons fn1 fns)
:collect (apply fn args))))
(defun rev (fn)
"Return a function that applies its arguments to FN in reverse order.
E.g. (funcall (rev #'cons) 1 2) ;; (2 . 1)"
(lambda (&rest args)
(apply fn (reverse args))))
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