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Trivia — Pattern Matching for Common Lisp

Table of Contents

Definition

trivia is a Common Lisp pattern-matching library. It provides match and ematch macros that dispatch on the structure and type of a value, binding sub-components to names in the matched branch.

Online documentation: github.com/guicho271828/trivia

Forms Used in Tolk

match

(match value
  (pattern₁ body₁)
  (pattern₂ body₂)
  (_ fallthrough-body))

Returns nil if no pattern matches (the _ catch-all prevents this).

ematch (exhaustive match)

(ematch value
  (pattern₁ body₁)
  (pattern₂ body₂))

Like match, but signals a trivia:match-error if no pattern matches. Tolk uses ematch throughout to get an error rather than silent nil on unrecognised input: 

(defmethod parse (sexp)
  (ematch sexp
    ((type number) (make-instance 'num :n sexp))
    (`(+ ,l ,r) (make-instance 'plus :l (parse l) :r (parse r)))
    (`(* ,l ,r) (make-instance 'mult :l (parse l) :r (parse r)))))

Patterns used in tolk

(type number)
Matches any value that satisfies (typep value 'number).
`(+ ,l ,r)
Quasiquote pattern (requires fare-quasiquote extension, see fare-quasiquote). Matches a three-element list whose first element is the symbol +, binding the second and third to l and r.
`(* ,l ,r)
Same for multiplication.

defmethod-bind and trivia

defmethod-bind uses bind from metabang-bind rather than trivia directly. Pattern matching in tolk's parsers uses ematch; method dispatch on AST node type uses CLOS (no trivia needed there).

Loading

(ql:quickload :trivia)             ; base
(ql:quickload :trivia.quasiquote)  ; quasiquote patterns (also loads fare-quasiquote)

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