fix(switch): evaluate argument only once

Switch.ark

@@ -1,3 +1,10 @@
+(macro _switch_impl (var case then ...cases) {
+  ($if (= "_" ($repr case))
+    then
+    (if (= var case)
+      then
+      ($if (>= ($len cases) 2) (_switch_impl var ...cases)))) })
+
 # @brief Takes a value to match against a list of (possible value, code to run)...
 # @param value value to match
 # @param case first case
@@ -14,8 +21,6 @@
 # =end
 # @author https://github.com/SuperFola
 (macro switch (value case then ...cases) {
-  ($if (= "_" ($repr case))
-    then
-    (if (= value case)
-      then
-      ($if (>= ($len cases) 2) (switch value ...cases)))) })
+  (macro var ($gensym))
+  (let var value)
+  (_switch_impl var case then ...cases) })

tests/switch-tests.ark

@@ -1,6 +1,14 @@
 (import std.Switch)
 (import std.Testing)
 
+(mut called 0)
+(let foo (fun () {
+  (set called (+ 1 called))
+  (if (> 1 called)
+    (test:expect false "foo shouldn't be evaluated more than once")
+    (test:expect true))
+  called }))
+
 (test:suite switch {
   (switch 12
     0  (test:expect false)
@@ -17,4 +25,15 @@
   (switch 1
     1 (test:expect true))
   (switch 1
-    2 (test:expect false)) })
+    2 (test:expect false))
+
+  (switch (foo)
+    0 (test:expect false)
+    1 (test:expect true)
+    2 (test:expect false)
+    _ (test:expect false))
+
+  (switch (let n 1)
+    0 (test:expect false)
+    1 (test:expect true)
+    _ (test:expect false)) })