Re: [PATCH] pcase.el: Add cl-type and type patterns

[Stefan Monnier <monnier@iro.umontreal.ca>]

> +@item (cl-type @var{type})
> +Matches if @var{expval} is of type @var{type}, which is a symbol or
> +list as accepted by @ref{cl-typep,,,cl,Common Lisp Extensions}.

Rather than mention "symbol or list", I'd only refer to `cl-typep`,
maybe something like:

    Matches if @var{expval} is of type @var{type}, which is type descriptor
    as accepted by @ref{cl-typep,,,cl,Common Lisp Extensions}.

The rest of the first patch looks good to me.

> +(pcase-defmacro type (type)
> +  "Pcase pattern that matches objects of TYPE.
> +This checks using a predicate named `TYPEp' or `TYPE-p', as
> +appropriate.  For matching structs defined with `cl-defstruct',
> +the `cl-type' pattern matcher should be used instead."
> +  (let* ((type (symbol-name type))
> +         (pred (or (intern-soft (concat type "p"))
> +                   (intern-soft (concat type "-p"))
> +                   (error "Unknown type: %s" type))))
> +    `(pred ,pred)))

I'm not convinced this second patch is worth the trouble:

    (type FOO)

is not significantly shorter or simpler than

    (pred FOO-p)

but it is inherently brittle because `intern-soft` may
return non-nil even though there's no predicate by that name, and it may
also let you use a "type" which really isn't one such as
`cl--struct-name` or `looking-at`.

`cl-type` provides that functionality in a cleaner and more
reliable way (it still also relies to some extent on similar heuristic
as your code, admittedly, but I've been working to get rid of it).


        Stefan