Implement subtraction of arrays. (#164)

Thanks to @kklingenberg for having provided the initial implementation of this functionality in jaq.

This should close #161.

jaq-interpret/src/val.rs

@@ -370,6 +370,11 @@ impl core::ops::Sub for Val {
             (Float(x), Float(y)) => Ok(Float(x - y)),
             (Num(n), r) => Self::from_dec_str(&n) - r,
             (l, Num(n)) => l - Self::from_dec_str(&n),
+            (Arr(mut l), Arr(r)) => {
+                let r = alloc::collections::BTreeSet::from_iter(r.iter());
+                Rc::make_mut(&mut l).retain(|x| !r.contains(x));
+                Ok(Arr(l))
+            }
             (l, r) => Err(Error::MathOp(l, MathOp::Sub, r)),
         }
     }

jaq-interpret/tests/tests.rs

@@ -43,6 +43,8 @@ fn sub() {
     give(json!(1.0), ". - 1", json!(0.0));
 }
 
+yields!(sub_arr, "[1, 2, 3] - [2, 3, 4]", json!([1]));
+
 #[test]
 fn mul() {
     give(json!(1), ". * 2", json!(2));