[metapost] input command

[Toby Thurston]

You are right that the Metapost manual (mpman.pdf) does explain the "input" operation properly.  

It is inherited directly from Metafont, so the best documentation for it is pages 179-180 of the Metafont Book. 
Fortunately it is pretty simple, so I can quote it directly here. The book says:

	- input <filename>.  When "input" is expanded, the expansion is null, but
    METAFONT prepares to read from the specified file before looking at any more 
    tokens from its current source.  A <filename> is subject to special restrictions
    explained on the next page.

And then on the next page it says that the syntax for <filename> depends on the local 
operating system and that MF does not even try to convert a file name to tokens; and hence
the operation must appear only in a text file, and not in a list of tokens like the replacement
text for a macro.  

The index in the MFBook makes it clear that "input" is a built-in token, and that therefore "it might
be dangerous to redefine it".  But re-definition is possible, as shown by the extended example on pp.287-288.

It might be helpful to have a summary of some of this in the MP documentation. 

Toby


> On 8 Apr 2023, at 11:00, metapost-request at tug.org wrote:
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> Today's Topics:
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>   1. input command (Ernst Reissner)
>   2. Re: input command (peter wilson)
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> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Fri, 7 Apr 2023 15:43:11 +0200
> From: Ernst Reissner <rei3ner at arcor.de>
> To: metapost at tug.org
> Subject: [metapost] input command
> Message-ID: <daf22203-7112-cb48-3f34-f8b3bd52efaf at arcor.de>
> Content-Type: text/plain; charset=UTF-8; format=flowed
> 
> Hello all,
> 
> I read the metapost manual by J.D. Hobby.
> 
> What I wonder: the 'command' input is used several times but it is 
> introduced nowhere.
> 
> What is it? a macro?, a command??
> 
> 
> Best Regards,
> 
> Ernst
> 
> 
> 
> ------------------------------
> 
> Message: 2
> Date: Fri, 7 Apr 2023 19:44:47 +0100
> From: peter wilson <herries.press at earthlink.net>
> To: metapost at tug.org
> Subject: Re: [metapost] input command
> Message-ID: <c799ab54-8a34-e660-335f-67209735ccfd at earthlink.net>
> Content-Type: text/plain; charset=UTF-8; format=flowed
> 
> Have you checked both the Metapost amd Metafun manuals? Have you asked 
> on the mailing list `https://lists.tug.org/metapost`? What do you think 
> `command` might mean? --- GOM
> 
> On 07/04/2023 14:43, Ernst Reissner wrote:
>> Hello all,
>> 
>> I read the metapost manual by J.D. Hobby.
>> 
>> What I wonder: the 'command' input is used several times but it is 
>> introduced nowhere.
>> 
>> What is it? a macro?, a command??
>> 
>> 
>> Best Regards,
>> 
>> Ernst
>> 
>> -- 
>> http://tug.org/metapost/
> 
> 
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