feat(tvix/eval): implement binary comparison operators r/4478

This is accomplished by simply delegating to the Rust implementations
of (Partial)Ord and (Partial)Eq, which are implemented for Value and
underlying wrapper types to behave like they do in Nix.

To ease the implementation overhead, a new comparison operator macro
has been added to the VM module.

Incomparable types will raise a new error variant when a comparison is
attempted, containing both supplied types. This mimics the information
carried in the error thrown by C++ Nix.

Change-Id: Ia19634d69119d40722f3ca672387bc3a80096998
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6143
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

1 files changed, 36 insertions, 7 deletions

diff --git a/tvix/eval/src/vm.rs b/tvix/eval/src/vm.rs
index db2f76d73145..30f2c5a4a414 100644
--- a/tvix/eval/src/vm.rs
+++ b/tvix/eval/src/vm.rs
@@ -18,15 +18,14 @@ pub struct VM {
 
 macro_rules! arithmetic_op {
     ( $self:ident, $op:tt ) => {{
-        let result = arithmetic_op!($self.pop(), $self.pop(), $op);
+        let b = $self.pop();
+        let a = $self.pop();
+        let result = arithmetic_op!(a, b, $op);
         $self.push(result);
     }};
 
-    ( $b:expr, $a:expr, $op:tt ) => {{
-        let b = $b;
-        let a = $a;
-
-        match (a, b) {
+    ( $a:ident, $b:ident, $op:tt ) => {{
+        match ($a, $b) {
             (Value::Integer(i1), Value::Integer(i2)) => Value::Integer(i1 $op i2),
             (Value::Float(f1), Value::Float(f2)) => Value::Float(f1 $op f2),
             (Value::Integer(i1), Value::Float(f2)) => Value::Float(i1 as f64 $op f2),
@@ -44,6 +43,31 @@ macro_rules! arithmetic_op {
     }};
 }
 
+macro_rules! cmp_op {
+    ( $self:ident, $op:tt ) => {{
+        let b = $self.pop();
+        let a = $self.pop();
+
+        // Comparable (in terms of ordering) values are numbers and
+        // strings. Numbers need to be coerced similarly to arithmetic
+        // ops if mixed types are encountered.
+        let result = match (a, b) {
+            (Value::Integer(i1), Value::Integer(i2)) => i1 $op i2,
+            (Value::Float(f1), Value::Float(f2)) => f1 $op f2,
+            (Value::Integer(i1), Value::Float(f2)) => (i1 as f64) $op f2,
+            (Value::Float(f1), Value::Integer(i2)) => f1 $op (i2 as f64),
+            (Value::String(s1), Value::String(s2)) => s1 $op s2,
+
+            (lhs, rhs) => return Err(Error::Incomparable {
+                lhs: lhs.type_of(),
+                rhs: rhs.type_of(),
+            }),
+        };
+
+        $self.push(Value::Bool(result));
+    }};
+}
+
 impl VM {
     fn inc_ip(&mut self) -> OpCode {
         let op = self.chunk.code[self.ip];
@@ -74,7 +98,7 @@ impl VM {
                     let result = if let (Value::String(s1), Value::String(s2)) = (&a, &b) {
                         Value::String(s1.concat(s2))
                     } else {
-                        arithmetic_op!(b, a, +)
+                        arithmetic_op!(a, b, +)
                     };
 
                     self.push(result)
@@ -107,6 +131,11 @@ impl VM {
                     self.push(Value::Bool(v1 == v2))
                 }
 
+                OpCode::OpLess => cmp_op!(self, <),
+                OpCode::OpLessOrEq => cmp_op!(self, <=),
+                OpCode::OpMore => cmp_op!(self, >),
+                OpCode::OpMoreOrEq => cmp_op!(self, >=),
+
                 OpCode::OpNull => self.push(Value::Null),
                 OpCode::OpTrue => self.push(Value::Bool(true)),
                 OpCode::OpFalse => self.push(Value::Bool(false)),