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11.4 Pattern-Matching Conditional

Aside from the four basic conditional forms, Emacs Lisp also has a pattern-matching conditional form, the pcase macro, a hybrid of cond and cl-case (see Conditionals in Common Lisp Extensions) that overcomes their limitations and introduces the pattern matching programming style. The limitations that pcase overcomes are:

  • The cond form chooses among alternatives by evaluating the predicate condition of each of its clauses (see Conditionals). The primary limitation is that variables let-bound in condition are not available to the clause’s body-forms.

    Another annoyance (more an inconvenience than a limitation) is that when a series of condition predicates implement equality tests, there is a lot of repeated code. (cl-case solves this inconvenience.)

  • The cl-case macro chooses among alternatives by evaluating the equality of its first argument against a set of specific values.

    Its limitations are two-fold:

    1. The equality tests use eql.
    2. The values must be known and written in advance.

    These render cl-case unsuitable for strings or compound data structures (e.g., lists or vectors). (cond doesn’t have these limitations, but it has others, see above.)

Conceptually, the pcase macro borrows the first-arg focus of cl-case and the clause-processing flow of cond, replacing condition with a generalization of the equality test which is a variant of pattern matching, and adding facilities so that you can concisely express a clause’s predicate, and arrange to share let-bindings between a clause’s predicate and body-forms.

The concise expression of a predicate is known as a pattern. When the predicate, called on the value of the first arg, returns non-nil, we say that “the pattern matches the value" (or sometimes “the value matches the pattern").

The pcase macro  Includes examples and caveats.
Extending pcase  Define new kinds of patterns.
Backquote-Style Patterns  Structural patterns matching.
Destructuring with pcase Patterns  Using pcase patterns to extract subfields.