Re: [PATCH 2/7] emacs: tag: allow default case in notmuch-tag-formats

[Austin Clements <amdragon@MIT.EDU>]

On Sat, 18 Jan 2014, Mark Walters <markwalters1009@gmail.com> wrote:
> Allow an empty string in notmuch-tag-formats which matches all tags
> except those matched explicitly matched. This allows the user to tell

Typo.

> notmuch to hide all tags except those specified.
>
> This will be useful once formatting for deleted/added tags is added
> later in the series: a user might want to hide all deleted tags for
> example.
> ---
>  emacs/notmuch-tag.el |   20 +++++++++++---------
>  1 files changed, 11 insertions(+), 9 deletions(-)
>
> diff --git a/emacs/notmuch-tag.el b/emacs/notmuch-tag.el
> index 2153068..92c1249 100644
> --- a/emacs/notmuch-tag.el
> +++ b/emacs/notmuch-tag.el
> @@ -65,14 +65,15 @@
>  This gives a list that maps from tag names to lists of formatting
>  expressions.  The car of each element gives a tag name and the
>  cdr gives a list of Elisp expressions that modify the tag.  If
> -the list is empty, the tag will simply be hidden.  Otherwise,
> -each expression will be evaluated in order: for the first
> -expression, the variable `tag' will be bound to the tag name; for
> -each later expression, the variable `tag' will be bound to the
> -result of the previous expression.  In this way, each expression
> -can build on the formatting performed by the previous expression.
> -The result of the last expression will displayed in place of the
> -tag.
> +the car is an empty string it matches all tags that do not have
> +an explicit match.  If the list is empty, the tag will simply be

Hmm.  I'm not sure I like overloading of the meanings of strings.  Could
we instead use a symbol to represent this case?  For example, `t' would
parallel the fall-through case of `cond' and `case', or `_' would
parallel `pcase' [1].  Or even a separate variable like
notmuch-tag-default-format?

The former would require some tweaking of the customize widget, but that
should probably happen anyway to support this special case.
Unfortunately, we may need a custom alist widget variant to do that.  (I
tried and failed to tweak it in a way that both worked and looked
decent.)  Hence my suggestion of a separate variable, which would only
require pulling out the :value-type into its own define-widget.

I'm also slightly bothered that this would introduce a second way to
control the default formatting of tags in addition to notmuch-tag-face,
but only slightly.

[1] It's unfortunate that pcase wasn't introduced until Emacs 24.  I've
been tempted to backport it for notmuch multiple times now.  Then we
could just treat notmuch-tag-formats as a list of pcase conditions.

> +hidden.  Otherwise, each expression will be evaluated in order:
> +for the first expression, the variable `tag' will be bound to the
> +tag name; for each later expression, the variable `tag' will be
> +bound to the result of the previous expression.  In this way,
> +each expression can build on the formatting performed by the
> +previous expression.  The result of the last expression will
> +displayed in place of the tag.
>  
>  For example, to replace a tag with another string, simply use
>  that string as a formatting expression.  To change the foreground
> @@ -140,7 +141,8 @@ This can be used with `notmuch-tag-format-image-data'."
>  
>  (defun notmuch-tag-format-tag (tag)
>    "Format TAG by looking into `notmuch-tag-formats'."
> -  (let ((formats (assoc tag notmuch-tag-formats)))
> +  (let ((formats (or (assoc tag notmuch-tag-formats)
> +		     (assoc "" notmuch-tag-formats))))
>      (cond
>       ((null formats)		;; - Tag not in `notmuch-tag-formats',
>        tag)			;;   the format is the tag itself.
> -- 
> 1.7.9.1
>
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