dds/utils.rkt

#lang typed/racket

;;; dds/utils

;;; Various utilities.

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

(provide auto-hash-ref/explicit auto-hash-ref/:)

;;; 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
;;;
(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.
(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))))