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syntax/parse: update comments

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This commit is contained in:
Ryan Culpepper 2018-07-30 14:20:43 +02:00
parent 6577791664
commit 7fb0250191
2 changed files with 29 additions and 4 deletions
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31
racket/collects/syntax/parse/private/opt.rkt
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@ -34,18 +34,43 @@
;; Can factor pattern P given clauses like
;; [ P P1 ... | e1] [ | [P1 ... | e1] ]
;; [ P ⋮ | ⋮] => [P | [ ⋮ | ⋮] ]
; [ P PN ... | eN] [ | [PN ... | eN] ]
;; [ P : | :] => [P | [ : | :] ]
;; [ P PN ... | eN] [ | [PN ... | eN] ]
;; if P cannot cut and P succeeds at most once (otherwise may reorder backtracking)
;; Can unfold pair patterns as follows:
;; [ (P11 . P12) P1 ... | e1 ] [ P11 P12 P1 ... | e1 ]
;; [ ⋮ ⋮ | ⋮ ] => check pair, [ ⋮ | ⋮ ]
;; [ : : | : ] => check pair, [ : | : ]
;; [ (PN1 . PN2) PN ... | eN ] [ PN1 PN2 PN ... | eN ]
;; Can unfold ~and patterns similarly; ~and patterns can hide
;; factoring opportunities.
;; ----
;; FIXME: New (unimplemented) optimization ideas
;; (1) When collecting pair patterns, can reorder rows with pair vs never-pair
;; first columns:
;; [ (P11 . P12) P1 ... | e1 ] [ (P11 . P12) P1 ... | e1 ]
;; [ P21 P2 ... | e2 ] => [ (P31 . P32) P3 ... | e3 ]
;; [ (P31 . P32) P3 ... | e3 ] [ P21 P2 ... | e2 ]
;; provided P21 does not cut and cannot match a pair term.
;; Likewise for literals and never-symbol patterns.
;; (2) If a row has a non-rejecting pattern (ie, always matches) in its first
;; column, then the rows above it do not need to produce failure information
;; *for their first columns*. For example, in the following matrix
;; [ P11 P1 ... | e1 ]
;; [ P21 P2 ... | e2 ]
;; [ P31 P3 ... | e3 ]
;; Suppose that P21 always matches (eg _) and assume P{1,3}1 are cut-free. Then
;; P{1,3}1 do not need to produce failure info (set es = #f, etc). Here's why.
;; If control reaches row 2, then since P21 cannot fail, if it fails the
;; progress must be greater than P11 or P31. FIXME: Must also check neither P11
;; nor P31 use ~post (or call stxclass that uses ~post, etc)!
;; ----
(define (optimize-matrix0 rows)

2
racket/collects/syntax/parse/private/parse.rkt
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@ -489,7 +489,7 @@ Conventions:
;; represents the matching matrix
;; [_in1_..._inN_|____]
;; [ P11 ... P1N | e1 ]
;; [ ⋮ ⋮ | ⋮ ]
;; [ : : | : ]
;; [ PM1 ... PMN | eM ]
(define-syntax (parse:matrix stx)