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Re: The poor state of documentation of pcase like things.

From: Phillip Lord
Subject: Re: The poor state of documentation of pcase like things.
Date: Mon, 21 Dec 2015 10:15:50 +0000
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

Richard Stallman <address@hidden> writes:

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> [[[ whether defending the US Constitution against all enemies,     ]]]
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>   > It matches anything (like a variable) but does not actually bind a 
> variable.
> Alas, that's the half of the answer that I already knew.
> What I don't know is, how do you _write_ the use of _?
> That seemed self-contradictory in the text that was posted.
> Can someone show me some properly working examples of _ in pcase
> and say what they do?

Okay. This is the simplest use and it returns 'hello

(pcase 1
  (_ 'hello))

It differs from a normal symbol because this:

(pcase 1
  (a a))

evals to 1, while this

(pcase 1
  (_ _))

errors with "void variable". That is `_' is not bound, unlike a. So `_'
is more than just convention.

Inside a list, though, it needs to be comma'd. So:

(pcase '(1 2 3)
  (`(,a 2 3) 'hello))

(pcase '(1 2 3)
  (`(,_ 2 3) 'hello))

both return `hello' (the former binding `a', the latter not binding

The question is how should this form behave.

(pcase '(1 2 3)
  (`(_ 2 3) 'hello))

Currently, it returns `nil' because `_' matches the symbol `_' not
anything. Obviously, this is useful if you want to match the symbol
`_'. But I think it's unintuitive.

Even if everyone agrees it is wrong, if it is fixable, I do not know. I
suspect it's probably too late in the day for the emacs-25.


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