Assert, Exit builtins

* Builtins for assert and exit are added
* Tests for assert are added
* file operations from previous MR are added to documents
* assert and exit are added to documents

src/stl/control.rs

@@ -17,9 +17,10 @@
 
 use crate::eval::eval;
 use crate::error::{Traceback, start_trace};
-use crate::segment::{Ctr, Seg};
+use crate::segment::{Ctr, Seg, Type};
 use crate::sym::{SymTable, Symbol, ValueType, Args};
 use std::rc::Rc;
+use std::process;
 
 const IF_DOCSTRING: &str =
     "accepts three bodies, a condition, an unevaluated consequence, and an alternative consequence.
@@ -381,7 +382,60 @@ fn circuit_callback(ast: &Seg, syms: &mut SymTable) -> Result<Ctr, Traceback> {
     }
 }
 
+const ASSERT_DOCSTRING: &str = "Takes one input: a boolean (or form that evaluates to a boolean).
+If input is false, a traceback is started and code throws an error.
+Otherwise, if input is true, returns None.";
+fn assert_callback(ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, Traceback> {
+    if let Ctr::Bool(b) = *ast.car {
+        if b {
+            Ok(Ctr::None)
+        } else {
+            Err(start_trace(("assert", "assertion failed").into()))
+        }
+    } else {
+        Err(start_trace(("assert", "impossible arg").into()))
+    }
+}
+
+const EXIT_DOCSTRING: &str = "Takes on input: an integer used as an exit code.
+Entire REPL process quits with exit code.";
+fn exit_callback(ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, Traceback> {
+    if let Ctr::Integer(i) = *ast.car {
+        if i > 2 ^ 32 {
+            panic!("argument to exit too large!")
+        } else {
+            process::exit(i as i32);
+        }
+    } else {
+        panic!("impossible argument to exit")
+    }
+}
+
 pub fn add_control_lib(syms: &mut SymTable) {
+    syms.insert(
+        "exit".to_string(),
+        Symbol {
+            name: String::from("exit"),
+            args: Args::Strict(vec![Type::Integer]),
+            conditional_branches: false,
+            docs: EXIT_DOCSTRING.to_string(),
+            value: ValueType::Internal(Rc::new(exit_callback)),
+            ..Default::default()
+        },
+    );
+
+    syms.insert(
+        "assert".to_string(),
+        Symbol {
+            name: String::from("assert"),
+            args: Args::Strict(vec![Type::Bool]),
+            conditional_branches: false,
+            docs: ASSERT_DOCSTRING.to_string(),
+            value: ValueType::Internal(Rc::new(assert_callback)),
+            ..Default::default()
+        },
+    );
+
     syms.insert(
         "if".to_string(),
         Symbol {