first pass at env variable component

ferretro_components/src/provided/stdlib/variables/env.rs

@@ -0,0 +1,34 @@
+use std::collections::BTreeMap;
+use crate::prelude::*;
+
+#[derive(Default)]
+pub struct EnvironmentVariableComponent {
+    variables: BTreeMap<String, String>,
+}
+
+impl RetroComponent for EnvironmentVariableComponent {}
+
+impl RetroCallbacks for EnvironmentVariableComponent {
+    fn get_variable(&mut self, key: &str) -> Option<String> {
+        self.variables.get(key).cloned()
+    }
+
+    fn set_variables(&mut self, variables: &[Variable2]) -> Option<bool> {
+        for v in variables {
+            println!("# {}", v.description);
+            let value = if let Ok(x) = std::env::var(&v.key) {
+                if !v.options.contains(&x) {
+                    panic!("Invalid value {:?} for variable {:?} (expected one of {:?})", x, v.key, v.options);
+                } else if x != v.options[0] {
+                    println!("# (overridden, default {})", v.options[0]);
+                }
+                x
+            } else {
+                v.options[0].clone()
+            };
+            println!("{}={} # {:?}", v.key, value, v.options);
+            self.variables.insert(v.key.to_owned(), value);
+        }
+        Some(true)
+    }
+}

ferretro_components/src/provided/stdlib/variables/mod.rs

@@ -0,0 +1,5 @@
+mod env;
+mod store;
+
+pub use env::EnvironmentVariableComponent;
+pub use store::VariableStoreComponent;