Global variables (#378)

* global variables

* implemented type checking of global variable, added test case

* added a comment

* improvements based on Dave's suggestions

* improvements based on Jon's suggestions

* added test cases about global variable ordering

executable_semantics/testdata/global_variable7.golden

@@ -0,0 +1,95 @@
+********** source program **********
+fn f () -> Int {
+return 0;
+
+}
+var Int : y = f()
+fn main () -> Int {
+return y;
+
+}
+********** type checking **********
+--- step exp Int --->
+--- step exp Int --->
+--- step exp Int --->
+--- step exp Int --->
+--- step exp Int --->
+--- step exp Int --->
+
+********** type checking complete **********
+fn f () -> Int {
+return 0;
+}
+var Int : y = f()
+fn main () -> Int {
+return y;
+}
+********** starting execution **********
+********** initializing globals **********
+--- step exp () --->
+--- step exp f() --->
+--- step exp f --->
+--- handle value fun<f> with f()<1>(fun<f>,) --->
+--- step exp () --->
+--- handle value () with f()<2>(fun<f>,(),) --->
+pattern_match((), ())
+--- step stmt return 0; --->
+--- step exp 0 --->
+--- handle value 0 with return 0;<1>(0,) --->
+--- step exp () --->
+********** calling main function **********
+{
+stack: top{main()<-1>}
+heap: fun<f>, 0, fun<main>, 
+env: main: fun<main>, y: 0, f: fun<f>, 
+}
+--- step exp main() --->
+{
+stack: top{main<-1> :: main()<0>}
+heap: fun<f>, 0, fun<main>, 
+env: main: fun<main>, y: 0, f: fun<f>, 
+}
+--- step exp main --->
+{
+stack: top{fun<main><-1> :: main()<0>}
+heap: fun<f>, 0, fun<main>, 
+env: main: fun<main>, y: 0, f: fun<f>, 
+}
+--- handle value fun<main> with main()<1>(fun<main>,) --->
+{
+stack: top{()<-1> :: main()<1>(fun<main>,)}
+heap: fun<f>, 0, fun<main>, 
+env: main: fun<main>, y: 0, f: fun<f>, 
+}
+--- step exp () --->
+{
+stack: top{()<-1> :: main()<1>(fun<main>,)}
+heap: fun<f>, 0, fun<main>, 
+env: main: fun<main>, y: 0, f: fun<f>, 
+}
+--- handle value () with main()<2>(fun<main>,(),) --->
+pattern_match((), ())
+{
+stack: main{return y;<-1>} :: top{}
+heap: fun<f>, 0, fun<main>, 
+env: main: fun<main>, y: 0, f: fun<f>, 
+}
+--- step stmt return y; --->
+{
+stack: main{y<-1> :: return y;<0>} :: top{}
+heap: fun<f>, 0, fun<main>, 
+env: main: fun<main>, y: 0, f: fun<f>, 
+}
+--- step exp y --->
+{
+stack: main{0<-1> :: return y;<0>} :: top{}
+heap: fun<f>, 0, fun<main>, 
+env: main: fun<main>, y: 0, f: fun<f>, 
+}
+--- handle value 0 with return y;<1>(0,) --->
+{
+stack: top{0<-1>}
+heap: fun<f>, 0, fun<main>, 
+env: main: fun<main>, y: 0, f: fun<f>, 
+}
+result: 0