Question

I’m using DrRacket as scheme compiler. When I’m trying to use the fold-right function, I got “fold-right: unbound identifier in module in: fold-right.” Do I need to include some library before calling this function?

Thank you!

Answer 1

I think you are looking for foldr. foldr is the racket implementation of fold-right.

From the documentation:

Like map, foldl applies a procedure to the elements of one or more
lists. Whereas map combines the return values into a list, foldl
combines the return values in an arbitrary way that is determined by
proc.

[…]

[foldr is ] like foldl, but the lists are traversed from right to
left. Unlike foldl, foldr processes the lsts in space proportional
to the length of lsts (plus the space for each call to proc).

Edit:

You may also want to use the srfi/1 library as suggested by Chris, particularily if you are expecting fold-right to work like the srfi/1 recommendation. There are some subtle differences:

#lang racket
(require srfi/1)
(foldr list* '() '(a b c) '(1 2 3))
(fold-right list* '() '(a b c) '(1 2 3 4))
(foldr list* '() '(a b c) '(1 2 3 4))

Output:

Welcome to DrRacket, version 5.3 [3m].
Language: racket; memory limit: 128 MB.
'{a 1 b 2 c 3}
'{a 1 b 2 c 3}
[ERROR] foldr: given list does not have the same size as the first list: '{1 2 3 4}

Note: fold-right does not error on different size lists, but you still only get the 1, 2, 3 atoms from the second list. This matches the srfi/1 spec:

The fold operation terminates when the shortest list runs out of
values

foldr does error.