utils: Start and auto-hash-ref/explicit.

Also add the tests for utils.

utils-tests.rkt

@@ -0,0 +1,16 @@
+#lang typed/racket
+
+;;; Tests for dds/utils.
+
+(require typed/rackunit "utils.rkt")
+
+(test-begin
+  (test-case "auto-hash-ref/explicit"
+    (let ([mytable #hash((a . 3) (b . 4))])
+      (check-equal? (auto-hash-ref/explicit (mytable b a)
+                                            (* a b))
+                    12))
+    (let ([ht #hash((a . #t) (b . #f))])
+      (check-equal? (auto-hash-ref/explicit (ht a b)
+                                            (and (not a) b))
+                    #f))))

utils.rkt

@@ -0,0 +1,27 @@
+#lang typed/racket
+
+;;; dds/utils
+
+;;; Various utilities.
+
+(require (for-syntax syntax/parse))
+
+(provide auto-hash-ref/explicit)
+
+;;; 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.
+(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)]))
+
+