Host Interface initial vars support

ablescript/src/host_interface.rs

@@ -1,5 +1,11 @@
+use crate::value::Variable;
+use std::collections::HashMap;
+
 /// Host Environment Interface
 pub trait HostInterface {
+    /// Initial variables for a stack frame
+    fn initial_vars() -> HashMap<String, Variable>;
+
     /// Print a string
     fn print(&mut self, string: &str, new_line: bool) -> std::io::Result<()>;
 
@@ -17,6 +23,10 @@ pub trait HostInterface {
 #[derive(Clone, Copy, Default)]
 pub struct Standard;
 impl HostInterface for Standard {
+    fn initial_vars() -> HashMap<String, Variable> {
+        HashMap::default()    
+    }
+
     fn print(&mut self, string: &str, new_line: bool) -> std::io::Result<()> {
         use std::io::Write;
 

ablescript/src/interpret.rs

@@ -91,7 +91,12 @@ impl<H: HostInterface> ExecEnv<H> {
     /// other information.
     pub fn with_host_interface(host_interface: H) -> Self {
         Self {
-            stack: vec![Default::default()],
+            stack: vec![
+                Default::default(),
+                Scope {
+                    variables: H::initial_vars(),
+                },
+            ],
             read_buf: Default::default(),
             finalisers: vec![],
             host_interface,
@@ -103,12 +108,15 @@ impl<H: HostInterface> ExecEnv<H> {
     where
         I: IntoIterator<Item = (String, Variable)>,
     {
-        let scope = Scope {
-            variables: ablescript_consts().into_iter().chain(vars).collect(),
-        };
-
         Self {
-            stack: vec![scope],
+            stack: vec![
+                Scope {
+                    variables: ablescript_consts().into_iter().chain(vars).collect(),
+                },
+                Scope {
+                    variables: H::initial_vars(),
+                },
+            ],
             read_buf: Default::default(),
             finalisers: vec![],
             host_interface,