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68 lines
2.5 KiB
Racket
68 lines
2.5 KiB
Racket
3 years ago
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#lang racket/base
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(require racket/contract/base)
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(provide
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(struct-out terminal-symbol)
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(struct-out nonterminal-symbol)
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(struct-out context-free-grammar)
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(struct-out context-free-production-rule)
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(contract-out
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[grammar-symbol? predicate/c]
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[grammar-start-rules
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(-> context-free-grammar? (set/c context-free-production-rule? #:kind 'immutable))]
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[make-grammar
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(-> #:rules (sequence/c context-free-production-rule?) #:start-symbol any/c context-free-grammar?)]
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[make-rule
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(-> #:symbol any/c #:substitution (sequence/c grammar-symbol?) #:label any/c
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context-free-production-rule?)]))
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(require racket/sequence
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racket/set
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rebellion/collection/vector)
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;@----------------------------------------------------------------------------------------------------
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;; Parsing takes a (Grammar T S L) and a sequence of (Token T V) and produces a set of
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;; (Parser-Derivation V L) (also called a "parse forest"). A grammar contains an immutable
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;; vector of (Context-Free-Production-Rule T S L) and a start symbol of type S.
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;; T: the terminals the grammar parses. Corresponds to the type field of the input tokens.
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;; S: the nonterminals the grammar rules are defined in terms of.
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;; L: the labels that grammar rules may have attached to them. These show up in parse tree
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;; branches, and can be used to determine which production rule produced a derivation.
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(struct context-free-grammar (rules start-symbol) #:transparent)
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(define (grammar-start-rules grammar)
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(define start (context-free-grammar-start-symbol grammar))
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(for/set ([rule (in-vector (context-free-grammar-rules grammar))]
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#:when (equal? (context-free-production-rule-nonterminal rule) start))
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rule))
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;; A (Context-Free-Production-Rule T S L) contains a nonterminal symbol of type S, a label of type L,
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;; and a substitution sequence of (Grammar-Symbol T S) values, stored in an immutable vector.
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(struct context-free-production-rule (nonterminal label substitution) #:transparent)
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;; A (Grammar-Symbol T S) is either a (Terminal-Symbol T) or a (Nonterminal-Symbol S)
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(define (grammar-symbol? v)
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(or (terminal-symbol? v) (nonterminal-symbol? v)))
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(struct terminal-symbol (value) #:transparent)
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(struct nonterminal-symbol (value) #:transparent)
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(define (make-grammar #:rules rules #:start-symbol start)
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(context-free-grammar (sequence->vector rules) start))
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(define (make-rule #:symbol symbol #:substitution substitution #:label label)
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(context-free-production-rule symbol label (sequence->vector substitution)))
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