add boolean cast function

src/stl/boolean.rs

@@ -117,3 +117,29 @@ pub fn toggle_callback(ast: &Seg, syms: &mut SymTable) -> Result<Ctr, String> {
     syms.insert(var_name, sym);
     Ok(Ctr::None)
 }
+
+
+pub const BOOLCAST_DOCSTRING: &str = "takes one argument of any type.
+attempts to cast argument to a bool.
+Strings will cast to a bool if they are 'true' or 'false'.
+Integers and Floats will cast to true if they are 0 and false otherwise.";
+
+pub fn boolcast_callback(ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, String> {
+    match &*ast.car {
+        Ctr::Bool(_) => Ok(*ast.car.clone()),
+        Ctr::Symbol(_) => Err("not clear how to cast a symbol to a bool".to_string()),
+        Ctr::String(s) => {
+            if s == "true" {
+                Ok(Ctr::Bool(true))
+            } else if s == "false" {
+                Ok(Ctr::Bool(false))
+            } else {
+                Err("string cannot be parsed as a bool".to_string())
+            }
+        },
+        Ctr::Integer(i) => Ok(Ctr::Bool(*i == 0)),
+        Ctr::Float(f) => Ok(Ctr::Bool(*f == 0.0)),
+        Ctr::Seg(_) => Err("cannot convert list to a boolean".to_string()),
+        Ctr::None => Err("Impossible task: cannot convert none to bool".to_string()),
+    }
+}