I'm starting to learn Emacs Lisp and as an exercise I'm trying to implement map using foldr. The code is the following:
(defun foldr (f z l)
(if (null l)
z
(funcall f (car l)
(foldr f z
(cdr l)))))
(defun map (f l)
(foldr (lambda (x z)
(cons (funcall f x) z))
nil
l))
However, when I try to evaluate, for example,
(map (lambda (x) (+ x 1)) '(1 2 3 4 5))
in Emacs with eval-last-sexp (after evaluating both foldr and map), the result is the following:
Debugger entered--Lisp error: (wrong-number-of-arguments (lambda (x z) (cons (funcall f x) z)) 1)
(lambda (x z) (cons (funcall f x) z))(5)
funcall((lambda (x z) (cons (funcall f x) z)) 5)
(cons (funcall f x) z)
(lambda (x z) (cons (funcall f x) z))(5 nil)
funcall((lambda (x z) (cons (funcall f x) z)) 5 nil)
(if (null l) z (funcall f (car l) (foldr f z (cdr l))))
foldr((lambda (x z) (cons (funcall f x) z)) nil (5))
(funcall f (car l) (foldr f z (cdr l)))
(if (null l) z (funcall f (car l) (foldr f z (cdr l))))
foldr((lambda (x z) (cons (funcall f x) z)) nil (4 5))
(funcall f (car l) (foldr f z (cdr l)))
(if (null l) z (funcall f (car l) (foldr f z (cdr l))))
foldr((lambda (x z) (cons (funcall f x) z)) nil (3 4 5))
(funcall f (car l) (foldr f z (cdr l)))
(if (null l) z (funcall f (car l) (foldr f z (cdr l))))
foldr((lambda (x z) (cons (funcall f x) z)) nil (2 3 4 5))
(funcall f (car l) (foldr f z (cdr l)))
(if (null l) z (funcall f (car l) (foldr f z (cdr l))))
foldr((lambda (x z) (cons (funcall f x) z)) nil (1 2 3 4 5))
map((lambda (x) (+ x 1)) (1 2 3 4 5))
eval((map (function (lambda (x) (+ x 1))) (quote (1 2 3 4 5))) nil)
eval-last-sexp-1(nil)
#[257 "\204\303!\207 \303!\n)B\211A =\204\211A\211@\207" [eval-expression-debug-on-error eval-last-sexp-fake-value debug-on-error eval-last-sexp-1] 4 2471606 "P"](nil)
ad-Advice-eval-last-sexp(#[257 "\204\303!\207 \303!\n)B\211A =\204\211A\211@\207" [eval-expression-debug-on-error eval-last-sexp-fake-value debug-on-error eval-last-sexp-1] 4 2471606 "P"] nil)
apply(ad-Advice-eval-last-sexp #[257 "\204\303!\207 \303!\n)B\211A =\204\211A\211@\207" [eval-expression-debug-on-error eval-last-sexp-fake-value debug-on-error eval-last-sexp-1] 4 2471606 "P"] nil)
eval-last-sexp(nil)
call-interactively(eval-last-sexp nil nil)
command-execute(eval-last-sexp)
I don't understand why this doesn't work, when the following super similar implementation in Haskell works fine:
foldr f z l = if (null l)
then z
else f (head l) (foldr f z (tail l))
map f l = foldr (\ x z -> (:) (f x) z) [] l
So why aren't the Lisp and Haskell programs equivalent? What's the nature of the problem in the Lisp implementation, i.e., why isn't map working?
(setq lexical-binding t)
f is a free variable in the lambda form you pass to foldr.
With lexical binding, your example works fine:
(map (lambda (x) (+ x 1)) '(1 2 3 4 5)) ; ==> (2 3 4 5 6)