Issue #336 - Validate num of args passed to if

CHANGELOG.md

@@ -7,6 +7,7 @@ https://github.com/cyclone-scheme/cyclone-winds
 
 Bug Fixes
 
+- Validate the number of arguments passed to `if` expressions.
 - Raise a useful error instead of aborting the whole program (!) when apply attempts to execute an object of the wrong type.
 - Better handling of edge cases where an object of the wrong type is executed instead of a closure. Previously there were cases where this would cause the runtime to crash.
 

scheme/eval.sld

@@ -547,13 +547,19 @@
     'ok))
 
 (define (analyze-if exp a-env rename-env local-renamed)
-  (let ((pproc (analyze (if-predicate exp) a-env rename-env local-renamed))
-        (cproc (analyze (if-consequent exp) a-env rename-env local-renamed))
-        (aproc (analyze (if-alternative exp) a-env rename-env local-renamed)))
-    (lambda (env)
-      (if (pproc env)
-          (cproc env)
-          (aproc env)))))
+  (let ((args (length exp)))
+    (cond
+      ((< args 3)
+       (error "Not enough arguments" exp))
+      ((> args 4)
+       (error "Too many arguments" exp)))
+    (let ((pproc (analyze (if-predicate exp) a-env rename-env local-renamed))
+          (cproc (analyze (if-consequent exp) a-env rename-env local-renamed))
+          (aproc (analyze (if-alternative exp) a-env rename-env local-renamed)))
+      (lambda (env)
+        (if (pproc env)
+            (cproc env)
+            (aproc env))))))
 
 (define (analyze-lambda exp a-env rename-env local-renamed)
   (let* ((vars (lambda-parameters exp))