dds/utils.rkt

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#lang typed/racket
;;; dds/utils
;;; Various utilities.
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(require (for-syntax syntax/parse)
(for-syntax racket/list))
(provide auto-hash-ref/explicit auto-hash-ref/: eval-with eval-with1)
;;; ===================
;;; HashTable Injection
;;; ===================
;;; This section of the file contains some utilities to streamline the
;;; usage of hash tables mapping symbols to values. The goal is
;;; essentially to avoid having to write explicit hash-ref calls.
;;; Given a (HashTable Symbol a) and a sequence of symbols, binds
;;; these symbols to the values they are associated to in the hash
;;; table, then puts the body in the context of these bindings.
;;;
;;; > (let ([ht #hash((a . 1) (b . 2))])
;;; (auto-hash-ref/explicit (ht a b) (+ a (* 2 b))))
;;; 5
;;;
;;; Note that only one expression can be supplied in the body.
(define-syntax (auto-hash-ref/explicit stx)
(syntax-parse stx
[(_ (ht:id xs:id ...) body:expr)
#`(let #,(for/list ([x (syntax->list #'(xs ...))])
#`[#,x (hash-ref ht '#,x)])
body)]))
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;;; Given an expression and a (HashTable Symbol a), looks up the
;;; symbols with a leading semicolon and binds them to the value they
;;; are associated to in the hash table.
;;;
;;; > (let ([ht #hash((a . 1) (b . 2))])
;;; (auto-hash-ref/: ht (+ :a (* 2 :b))))
;;; 5
;;;
;;; Note that the symbol :a is matched to the key 'a in the hash
;;; table.
;;;
;;; Note that only one expression can be supplied in the body.
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(define-syntax (auto-hash-ref/: stx)
(syntax-parse stx
[(_ ht:id body)
(let* ([names/: (collect-colons (syntax->datum #'body))])
#`(let #,(for/list ([x names/:])
;; put x in the same context as body
#`[#,(datum->syntax #'body x)
(hash-ref ht '#,(strip-colon x))])
body))]))
;;; The helper functions for auto-hash-ref/:.
(begin-for-syntax
;; Collect all the symbols starting with a colon in datum.
(define (collect-colons datum)
(remove-duplicates
(flatten
(for/list ([token datum])
(cond
[(symbol? token)
(let ([name (symbol->string token)])
(if (eq? #\: (string-ref name 0))
token
'()))]
[(list? token)
(collect-colons token)]
[else '()])))))
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;; Strip the leading colon off x.
(define (strip-colon x)
(let ([x-str (symbol->string x)])
(if (eq? #\: (string-ref x-str 0))
(string->symbol (substring x-str 1))
x))))
;;; Temporarily injects the mappings from the given hash table as
;;; bindings in the current namespace and then evaluates the
;;; expression.
;;;
;;; > (let ([ht #hash((a . 1) (b . 1))])
;;; (eval-with ht '(+ b a 1)))
;;; 3
;;;
;;; The local bindings from the current lexical scope are not
;;; conserved. Therefore, the following outputs an error about a
;;; missing identifier:
;;;
;;; > (let ([ht #hash((a . 1) (b . 1))]
;;; [z 1])
;;; (eval-with ht '(+ b z a 1)))
;;;
(: eval-with (-> (HashTable Symbol Any) Any AnyValues))
(define (eval-with ht expr)
(parameterize ([current-namespace (current-namespace)])
(hash-for-each ht (lambda (x val)
(namespace-set-variable-value! x val)))
(eval expr)))
;;; Same as eval-with, but returns only the first value produced by
;;; the evaluated expression.
(: eval-with1 (-> (HashTable Symbol Any) Any Any))
(define (eval-with1 ht expr)
(let ([vals (call-with-values (λ () (eval-with ht expr))
(λ vals vals))])
(car vals)))