Fix temporaries breaking (let ...) blocks, (gensym)

examples/debug.lysp

@@ -1 +0,0 @@
-(print (explain explain))

examples/macros.lysp

@@ -19,14 +19,16 @@
 (assert (= '(2 3 4 2 3 4) `(,@glob1 ,@glob1)))
 (assert (= '((((2 3 4)))) `(((,glob1)))))
 
-(defmacro debug (expression)
-  (print "DEBUG:" expression)
-  expression
-  )
-
 ; those are prelude, but defined in lysp itself:
 
-(debug
+(print "The previously printed expression is AFTER this one in the code")
+(compile-debug
+  (when #t
+    (print "a")
+    (print "b")
+    )
+  )
+(runtime-debug
   (when #t
     (print "a")
     (print "b")
@@ -43,3 +45,11 @@
   )
 
 
+; catch macro
+(print "Failing in a catch block...")
+(catch
+  (print a)
+  e (print "Caught an error:" e)
+  )
+
+(print "... doesn't fail the execution")

src/compile/block.rs

@@ -267,6 +267,29 @@ impl CompileContext {
         }
     }
 
+    pub fn guard_argument(&mut self) {
+        let depth = self.scope_depth as isize;
+        self.locals.push(Local {
+            name: "".into(),
+            depth,
+            is_captured: false,
+        });
+    }
+
+    pub fn unguard_argument(&mut self) {
+        let guard = self
+            .locals
+            .pop()
+            .expect("unguard_argument() called with an empty locals stack");
+        let stack_index = self.locals.len();
+        if guard.name.as_ref() != "" {
+            panic!(
+                "unguard_argument(): the local is not a guard: {:?} at stack index {}",
+                guard.name, stack_index,
+            );
+        }
+    }
+
     pub fn push_scope(&mut self) {
         self.scope_depth += 1;
         if self.options.trace_compile {

src/compile/codegen/function.rs

@@ -53,13 +53,22 @@ impl Compile for CallExpression {
         if instruction == Instruction::Call {
             let callee = self.callee.compile(cx)?;
             cx.push(callee)?;
+            cx.guard_argument();
         }
         for expression in self.arguments.iter() {
             let value = expression.compile(cx)?;
             cx.push(value)?;
+            cx.guard_argument();
         }
         cx.emit(instruction);
         cx.emit(argument_count);
+        let mut unguard_count = usize::from(argument_count);
+        if instruction == Instruction::Call {
+            unguard_count += 1;
+        }
+        for _ in 0..unguard_count {
+            cx.unguard_argument();
+        }
         Ok(CompileValue::Stack)
     }
 }

src/compile/syntax/mod.rs

@@ -121,7 +121,7 @@ impl Expression {
                     _ => Self::map_or(CallExpression::parse(cons, value), Expression::Call),
                 }
             }
-            Value::Keyword(_) => todo!("Error here"),
+            Value::Keyword(keyword) => todo!("Error here: keyword {keyword}"),
             Value::Closure(_) => todo!("Error here"),
             Value::Function(_) => todo!("Error here"),
             Value::NativeFunction(_) => todo!("Error here"),

src/prelude.lysp

@@ -1,8 +1,70 @@
-(defmacro when (condition &rest body)
-  `(if ,condition (progn ,@body))
+; Convenience flow control macros
+(defmacro when (condition body-head &rest body)
+  "If condition is true, evaluates the expressions in body, otherwise returns nil"
+  `(if ,condition (progn ,body-head ,@body))
   )
 
-(defmacro unless (condition &rest body)
-  `(if (not ,condition) (progn ,@body))
+(defmacro unless (condition body-head &rest body)
+  "If condition is false, evaluates the expressions in body, otherwise returns nil"
+  `(if (not ,condition) (progn ,body-head ,@body))
   )
 
+; Result handling functions
+(defun result/ok? (x)
+  "Returns #t if x is an ok"
+  (= 'ok (car x))
+  )
+(defun result/err? (x)
+  "Returns #t if x is an error"
+  (= 'err (car x))
+  )
+(defun result/map (f x)
+  "If x is an ok, applies f to its value, otherwise does nothing"
+  (if (result/ok? x)
+    `(ok ,(f (cadr x)))
+    x
+    )
+  )
+(defun result/map-err (f x)
+  "If x is an error, applies f to its value, otherwise does nothing"
+  (if (result/err? x)
+    `(err ,(f (cadr x)))
+    x
+    )
+  )
+
+; Convenience error handling macros
+(defmacro catch (expression error-symbol handler)
+  "Evaluates expression, running the handler if evaluation fails, introducing error-symbol as error"
+  (let (eval-result-symbol (gensym))
+    `(let (,eval-result-symbol (eval (quote ,expression)))
+       (cond
+         ((result/ok? ,eval-result-symbol) (cadr ,eval-result-symbol))
+         (&otherwise
+           (let (,error-symbol (cadr ,eval-result-symbol))
+             ,handler
+             )
+           )
+         )
+       )
+    )
+  )
+
+(defmacro compile-debug (expression)
+  "Prints the input expression during macro expansion/compile time and evaluates the expression in runtime"
+  (print expression)
+  expression
+  )
+(defmacro runtime-debug (expression)
+  "Prints the input expression and evaluates it in runtime"
+  `(progn
+     (print (quote ,expression))
+     ,expression
+     )
+  )
+
+; Convenience list functions
+(defun cadr (x) "Alias for (car (cdr x))" (car (cdr x)))
+(defun cdar (x) "Alias for (cdr (car x))" (cdr (car x)))
+(defun caddr (x) "Alias for (car (cdr (cdr x)))" (car (cdr (cdr x))))
+(defun cadar (x) "Alias for (car (cdr (car x)))" (car (cdr (car x))))

src/vm/env.rs

@@ -1,4 +1,12 @@
-use std::{borrow::Borrow, cell::RefCell, collections::HashMap, fmt, hash::Hash, rc::Rc};
+use std::{
+    borrow::Borrow,
+    cell::RefCell,
+    collections::HashMap,
+    fmt,
+    hash::Hash,
+    rc::Rc,
+    sync::atomic::{AtomicU32, Ordering},
+};
 
 use crate::{
     error::MachineError,
@@ -22,6 +30,7 @@ pub struct Environment {
     globals: RefCell<HashMap<IdentifierValue, Value>>,
     macros: RefCell<HashMap<IdentifierValue, Macro>>,
     parent: Option<Rc<Environment>>,
+    gensym_index: AtomicU32,
 }
 
 impl Environment {
@@ -32,6 +41,12 @@ impl Environment {
         }
     }
 
+    pub fn gensym(&self) -> IdentifierValue {
+        // TODO do this in a root environment?
+        let index = self.gensym_index.fetch_add(1, Ordering::SeqCst);
+        format!("___gensym{index}").into()
+    }
+
     pub fn defun_native<S, D, F>(&self, identifier: S, docstring: D, function: F) -> Value
     where
         S: Into<IdentifierValue>,

src/vm/prelude/debug.rs

@@ -6,6 +6,9 @@ use crate::{
 };
 
 pub fn load(env: &Rc<Environment>) {
+    env.defun_native("gensym", "Provides an unique symbol name", |_, env, _| {
+        Ok(env.gensym().into())
+    });
     env.defun_native(
         "explain",
         "Provides an explanation for a given value",

src/vm/value/convert.rs

@@ -7,8 +7,8 @@ use crate::{
         env::Environment,
         machine::Machine,
         value::{
-            BooleanValue, BytecodeFunction, ClosureValue, ConsCell, NativeFunction, NativeValue,
-            NumberValue, StringValue, Vector,
+            BooleanValue, BytecodeFunction, ClosureValue, ConsCell, IdentifierValue,
+            NativeFunction, NativeValue, NumberValue, StringValue, Vector,
         },
     },
 };
@@ -126,6 +126,12 @@ impl From<Vec<u8>> for Value {
     }
 }
 
+impl From<IdentifierValue> for Value {
+    fn from(value: IdentifierValue) -> Self {
+        Value::Identifier(value)
+    }
+}
+
 impl TryFromValue<'_> for AnyFunction {
     fn try_from_value(value: &'_ Value) -> Result<Self, ValueConversionError> {
         match value {

src/vm/value/keyword.rs

@@ -52,6 +52,7 @@ impl_keyword! {
         Break => "break",
         Continue => "continue",
         Declare => "declare",
+        Error => "&error",
         // Cons => "cons",
     }
 }