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Extending pcase

The pcase macro supports several kinds of patterns (see Pattern-Matching Conditional). You can add support for other kinds of patterns using the pcase-defmacro macro.

Macro: pcase-defmacro name args [doc] &rest body

Define a new kind of pattern for pcase, to be invoked as (name actual-args). The pcase macro expands this into a function call that evaluates body, whose job it is to rewrite the invoked pattern into some other pattern, in an environment where args are bound to actual-args.

Additionally, arrange to display doc along with the docstring of pcase. By convention, doc should use EXPVAL to stand for the result of evaluating expression (first arg to pcase).

Typically, body rewrites the invoked pattern to use more basic patterns. Although all patterns eventually reduce to core patterns, body need not use core patterns straight away. The following example defines two patterns, named less-than and integer-less-than.

(pcase-defmacro less-than (n)
  "Matches if EXPVAL is a number less than N."
  `(pred (> ,n)))

(pcase-defmacro integer-less-than (n)
  "Matches if EXPVAL is an integer less than N."
  `(and (pred integerp)
        (less-than ,n)))

Note that the docstrings mention args (in this case, only one: n) in the usual way, and also mention EXPVAL by convention. The first rewrite (i.e., body for less-than) uses one core pattern: pred. The second uses two core patterns: and and pred, as well as the newly-defined pattern less-than. Both use a single backquote construct (see Backquote).