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Re: Some help needed to use curl lib to download binary file

From: Sébastien Rey-Coyrehourcq
Subject: Re: Some help needed to use curl lib to download binary file
Date: Mon, 01 Aug 2022 09:48:47 +0200
User-agent: mu4e 1.6.11; emacs 28.1.90

Thanks a lot for this very detailled explanation Vivien, that’s help me a lot 
to understand what happens here !!

I’m trying to continue my learning of Lisp/Scheme like language, this is a new 
fascinating world to explore :)

Best regards,

Vivien Kraus <> writes:

> Hello,
> I see in the paste:
>> ;; function taken on
>> <>
>> ;; then adapted to use cookie jar
>> (define-public (http-get url cookie-exist)
>>   ;; Create a Curl handle
>>   (let ((handle (curl-easy-init)))
>>     ;; Set the URL from which to get the data
>>     (curl-easy-setopt handle ’url url)
>>     (if cookie-exist
>>         (curl-easy-setopt handle ’cookie “cookie.txt”)
>>         (curl-easy-setopt handle ’cookiejar “cookie.txt”))
>>     ;; Request that the HTTP headers be included in the response
>>     (curl-easy-setopt handle ’header #t)
>>     ;; Get the result as a Latin-1 string
>>     (let* ((response-string (curl-easy-perform handle))
>>            ;; Create a string port from the response
>>            (response-port (open-input-string response-string))
>>            ;; Have the (web response) module to parse the response
>>            (response (read-response response-port))
>>            (body (utf8->string (read-response-body response))))
>>       (close response-port)
>>       ;; Have the (web response) module extract the body from the
>>       ;; response
>>       (values response body))))
> So here the call expects the response to be UTF-8 text. If it is a
> binary file that you are downloading, the function will raise an
> exception. Guile has that python3 feeling where you are supposed to
> know in advance whether what you are using is text or binary, which is
> hurting you here. 
> However, you can avoid the problem by either having bytevectors
> everywhere, so removing the call to utf8->string and bind the “body”
> variable directly to (read-response-body response), or pretend that you
> know better than guile and pretend that it is latin-1-encoded, so you
> lose no information and guile won’t complain. In that case, load (ice-9
> iconv) and replace (utf8->string …) with (bytevector->string … “ISO-
> 8859-1”).
> If you go the first route, you get a bytevector back. If you go the
> second one, you get a string, but you must remember that it contains
> raw bytes and not text (unless the response body was indeed text).
> There are some cases when you might want to have such strings-that-
> contain-binary-or-text, such as if you want to use the strings API on
> them and the bytevector API does not provide what you want, or you want
> to interface with NUL-terminated strings. In other cases you might
> prefer bytevectors. Anyway, there is no encoding cost to convert
> between strings-that-contain-binary-or-toxt and bytevectors, it is as
> simple as a copy.
>> ;; write content into file
>> (call-with-output-file “” (lambda (current-output-port)
>>                                         (get-file get-file-link)
>>                                         (put-bytevector (current-
>> output-port) body)))
> call-with-output-file calls its function argument with a port. Plus,
> you just ignore the result of get-file, so instead you should do
> something with it.
> If you chose to have get-file return a bytevector, you can do:
> (call-with-output-file “”
>   (lambda (port)
>     (put-bytevector port (get-file get-file-link)))
>   #:binary #t)
> If get-file returns a string, you can do:
> (call-with-output-file “”
>   (lambda (port)
>     (put-string port (get-file get-file-link)))
>   #:encoding “ISO-8859-1”)
> Best regards,
> Vivien

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