Add tail-sharing-cond-to-when rule (#618)

Author: jackfirth

default-recommendations/conditional-shortcuts-test.rkt

@@ -1088,3 +1088,32 @@ test: "cond in ignored context with single clause refactorable"
     [else (void)])
   42)
 --------------------
+
+
+test: "cond with shared tail expression refactorable to when"
+--------------------
+(define (f c1)
+  (cond
+    [c1
+     (displayln "condition 1")
+     (displayln "shared tail")]
+    [else
+     (displayln "shared tail")]))
+====================
+(define (f c1)
+  (when c1
+    (displayln "condition 1"))
+  (displayln "shared tail"))
+--------------------
+
+
+no-change-test: "cond with unshared tail expression not refactorable to when"
+--------------------
+(define (f c1)
+  (cond
+    [c1
+     (displayln "condition 1")
+     (displayln "true tail")]
+    [else
+     (displayln "false tail")]))
+--------------------

default-recommendations/conditional-shortcuts.rkt

@@ -16,6 +16,7 @@
          resyntax/default-recommendations/private/exception
          resyntax/default-recommendations/private/let-binding
          resyntax/default-recommendations/private/metafunction
+         resyntax/default-recommendations/private/syntax-equivalence
          syntax/parse)
 
 
@@ -276,6 +277,21 @@
   (cond-id clause ... [else (void)]))
 
 
+(define-definition-context-refactoring-rule tail-sharing-cond-to-when
+  #:description "Both branches of this `cond` expression share the same tail expression. Moving that \
+tail expression outside `cond` lets you replace `cond` with `when`."
+  #:literals (cond else)
+  (~seq body-before ...
+        (cond [condition:expr true-body ... true-tail]
+              [else false-tail])
+        body-after ...)
+  #:when (syntax-free-identifier=? (attribute true-tail) (attribute false-tail))
+  (body-before ...
+   (when condition true-body ...)
+   true-tail
+   body-after ...))
+
+
 (define-refactoring-suite conditional-shortcuts
   #:rules (always-throwing-cond-to-when
            always-throwing-if-to-when
@@ -292,4 +308,5 @@
            ignored-and-to-when
            nested-if-to-cond
            nested-when-to-compound-when
+           tail-sharing-cond-to-when
            throw-unless-truthy-to-or))

default-recommendations/private/syntax-equivalence.rkt

@@ -27,7 +27,7 @@
   (define other-datum (syntax-e other-stx))
   (match datum
     [(? symbol?) (and (symbol? other-datum) (free-identifier=? stx other-stx))]
-    [(? number? boolean? string?) (equal? datum other-datum)]
+    [(or (? number?) (? boolean?) (? string?)) (equal? datum other-datum)]
     [(? list?) (and (list? other-datum) (syntax-pair-free-identifier=? datum other-datum))]
     [(? box?) (and (box? other-datum) (syntax-free-identifier=? (unbox datum) (unbox other-datum)))]
     [(? vector?)