add isset

Signed-off-by: Ava Hahn <ava@aidanis.online>

src/stl.rs

@@ -162,6 +162,17 @@ pub fn static_stdlib(syms: &mut SymTable) -> Result<(), String> {
         },
     );
 
+    syms.insert(
+        "set?".to_string(),
+        Symbol {
+            name: String::from("set?"),
+            args: Args::Strict(vec![Type::Symbol]),
+            conditional_branches: true,
+            docs: ISSET_DOCSTRING.to_string(),
+            value: ValueType::Internal(Rc::new(_isset_callback)),
+        },
+    );
+
     Ok(())
 }
 
@@ -356,3 +367,22 @@ fn _store_callback(ast: &Seg, syms: &mut SymTable, env_cfg: bool) -> Result<Ctr,
     }
     Ok(Ctr::None)
 }
+
+pub const ISSET_DOCSTRING: &str = "accepts a single argument: a symbol.
+returns true or false according to whether or not the symbol is found in the symbol table.";
+
+fn _isset_callback(ast: &Seg, syms: &mut SymTable) -> Result<Ctr, String> {
+    if ast.len() != 1 {
+        Err("help only takes a single argument".to_string())
+    } else {
+        if let Ctr::Symbol(ref symbol) = *ast.car {
+            if let Some(_) = syms.get(symbol) {
+                Ok(Ctr::Bool(true))
+            } else {
+                Ok(Ctr::Bool(false))
+            }
+        } else {
+            Err("help should only be called on a symbol".to_string())
+        }
+    }
+}