dds/utils.rkt

#lang racket

;;; dds/utils

;;; Various utilities.

(require (for-syntax syntax/parse)
         (for-syntax racket/list))

(provide auto-hash-ref/explicit auto-hash-ref/:)
(provide (contract-out [eval-with (-> hash? any/c any)]
                       [eval-with1 (-> hash? any/c any/c)]
                       [extract-symbols (-> any/c list?)]))

;;; ===================
;;; 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)]))

;;; 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.
(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 '()])))))

  ;; 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 a namespace including racket/base 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)))
;;;
(define (eval-with ht expr)
  (parameterize ([current-namespace (make-base-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.
(define (eval-with1 ht expr)
  (let ([vals (call-with-values (λ () (eval-with ht expr))
                                (λ vals vals))])
    (car vals)))


;;; ==============================
;;; Analysis of quoted expressions
;;; ==============================

;;; Produces a list of symbols appearing in the quoted expression
;;; passed in the first argument.
(define (extract-symbols form)
  (cond
    [(symbol? form)
     (list form)]
    [(list? form)
     (flatten (for/list ([x form]) (extract-symbols x)))]
    [else '()]))