call system executables from undefined function calls

ast/eval.go

@@ -19,6 +19,9 @@ package ast
 
 import "gitlab.com/whom/shs/log"
 
+var CallExecutablesFromUndefFuncCalls = false
+var CallExecutableToken = "l"
+
 func (t *Token) Eval(funcs FuncTable, vars VarTable) (*Token, bool) {
     if t == nil {
         return nil, false
@@ -69,13 +72,29 @@ func (t *Token) Eval(funcs FuncTable, vars VarTable) (*Token, bool) {
         return nil, false
     }
 
+    //if symbol in front of a list, could be a function call
     if ret.Tag == SYMBOL {
         f := GetFunction(ret.Inner.(string), funcs)
         if f == nil {
-            log.Log(log.DEBUG,
-                "could not find definition for symbol " + ret.Inner.(string),
-                "eval")
-            return nil, false
+            if CallExecutablesFromUndefFuncCalls {
+                f = GetFunction(CallExecutableToken, funcs)
+                if f == nil {
+                    log.Log(log.DEBUG, "Symbol " + ret.Inner.(string) + 
+                        " has no definition in function table. Additionally " +
+                        "the configured LoadExecutableToken is also not defined",
+                        "eval")
+                    return ret, false
+                }
+
+                // see the use of CallFunction below
+                ret = &Token{Next: ret}
+
+            } else {
+               log.Log(log.DEBUG,
+                    "could not find definition for symbol " + ret.Inner.(string),
+                    "eval")
+                return ret, false
+            }
         }
 
         return (*f).CallFunction(ret.Next, vars, funcs), true