Allow with modifier for builtin and user functions (#42)

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

src/builtins/mod.rs

@@ -13,6 +13,7 @@ pub mod numbers;
 mod objects;
 pub mod sets;
 mod strings;
+mod time;
 mod tracing;
 pub mod types;
 mod utils;
@@ -51,7 +52,7 @@ lazy_static! {
 	encoding::register(&mut m);
 	//token_signing::register(&mut m);
 	//token_verification::register(&mut m);
-	//time::register(&mut m);
+	time::register(&mut m);
 	//cryptography::register(&mut m);
 	//graphs::register(&mut m);
 	//graphql::register(&mut m);

src/builtins/time.rs

@@ -0,0 +1,30 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+use crate::ast::Expr;
+use crate::builtins;
+use crate::builtins::utils::ensure_args_count;
+use crate::lexer::Span;
+use crate::value::Value;
+
+use std::collections::HashMap;
+use std::time::SystemTime;
+
+use anyhow::{bail, Result};
+
+pub fn register(m: &mut HashMap<&'static str, builtins::BuiltinFcn>) {
+    m.insert("time.now_ns", (now_ns, 0));
+}
+
+fn now_ns(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
+    let name = "time.now_ns";
+    ensure_args_count(span, name, params, args, 0)?;
+
+    let now = SystemTime::now();
+    let elapsed = match now.duration_since(SystemTime::UNIX_EPOCH) {
+        Ok(e) => e,
+        Err(e) => bail!(span.error(format!("could not fetch elapsed time. {e}").as_str())),
+    };
+    let nanos = elapsed.as_nanos();
+    Ok(Value::from_u128(nanos))
+}