Help in understanding 'letrec' example

[Utkarsh Singh]

Hi,

First of all I would like to thank Racket community for creating and
maintaining top quality documentation at https://docs.racket-lang.org/
and even providing a local copy for it.

Currently I am having some difficulties in understanding this letrec
example from Racket Guide docs
(https://docs.racket-lang.org/guide/let.html#%28part._.Recursive_.Binding__letrec%29):

(letrec ([tarzan-near-top-of-tree?
(lambda (name path depth)
(or (equal? name "tarzan")
(and (directory-exists? path)
(tarzan-in-directory? path depth))))]
[tarzan-in-directory?
(lambda (dir depth)
(cond
[(zero? depth) #f]
[else
(ormap
(λ (elem)
(tarzan-near-top-of-tree? (path-element->string elem)
(build-path dir elem)
(- depth 1)))
(directory-list dir))]))])
(tarzan-near-top-of-tree? "tmp"
(find-system-path 'temp-dir)
4))

Problem:
I having some problem on how recursion is working here and what is the
problem we are solving here. Are we finding a file with (name?
"tarzan") or something else?

--
Utkarsh Singh
http://utkarshsingh.xyz

[kalime...@gmail.com]

We are finding a file (or directory) with name "tarzan" inside all directories inside given path upto given depth.

Recursion is needed here, because  tarzan-near-top-of-tree? calls  tarzan-in-directory? and  tarzan-in-directory? calls  tarzan-near-top-of-tree? for each file in given directory.

суббота, 8 мая 2021 г. в 12:50:09 UTC+5, Utkarsh Singh:

[Utkarsh Singh]

Hi,

> We are finding a file (or directory) with name "tarzan" inside all
> directories inside given path upto given depth.
>
> Recursion is needed here, because tarzan-near-top-of-tree? calls
> tarzan-in-directory? and tarzan-in-directory? calls
> tarzan-near-top-of-tree? for each file in given directory.

Thanks! I finally understood it.

Here is the explanation (for reference):

Consider a sample directory called test with following structure (ASCII
art using 'tree' command):

test
├── dir1
│   └── subdir1
│   └── file1
└── dir2

Our code will first recursively search for all file in dir1 upto depth 4
and then move to dir2. Using ormap insures that search for 'tarzan'
will stop at first match (or first true result).

[David Storrs]

Incidentally, a more concise way of doing this would be:

(define target (build-path "tarzan")) ; convert to path only once
(for/or ([item (in-directory "/tmp/test")]) ; or whatever directory you want to start in
  (equal? target (file-name-from-path item)))

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[Utkarsh Singh]

Hi David,

On 2021-05-13, 13:12 -0400, David Storrs <david....@gmail.com> wrote:

> Incidentally, a more concise way of doing this would be:
>
> (define target (build-path "tarzan")) ; convert to path only once
> (for/or ([item (in-directory "/tmp/test")]) ; or whatever directory
> you want to start in
> (equal? target (file-name-from-path item)))

Thanks! This looks good.