mod function

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

src/builtins/sets.rs

@@ -0,0 +1,26 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+use crate::ast::Expr;
+use crate::value::Value;
+
+use std::collections::BTreeSet;
+use std::rc::Rc;
+
+use anyhow::{bail, Result};
+
+fn ensure_set(fcn: &str, arg: &Expr, v: Value) -> Result<Rc<BTreeSet<Value>>> {
+    Ok(match v {
+        Value::Set(s) => s,
+        _ => {
+            let span = arg.span();
+            bail!(span.error(format!("`{fcn}` expects set argument. Got `{v}` instead").as_str()))
+        }
+    })
+}
+
+pub fn difference(expr1: &Expr, expr2: &Expr, v1: Value, v2: Value) -> Result<Value> {
+    let s1 = ensure_set("difference", expr1, v1)?;
+    let s2 = ensure_set("difference", expr2, v2)?;
+    Ok(Value::from_set(s1.difference(&s2).cloned().collect()))
+}