Re: input command

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, [email protected] 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 <[email protected]>
> To: [email protected]
> Subject: [metapost] input command
> Message-ID: <[email protected]>
> 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 <[email protected]>
> To: [email protected]
> Subject: Re: [metapost] input command
> Message-ID: <[email protected]>
> 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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> End of metapost Digest, Vol 179, Issue 2
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