tracing::trace builtin

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

src/builtins/mod.rs

@@ -9,6 +9,7 @@ mod debugging;
 pub mod numbers;
 pub mod sets;
 mod strings;
+mod tracing;
 pub mod types;
 mod utils;
 
@@ -56,7 +57,7 @@ lazy_static! {
 	//rego::register(&mut m);
 	//opa::register(&mut m);
 	debugging::register(&mut m);
-	//tracing::register(&mut m);
+	tracing::register(&mut m);
 	
 	m
     };

src/builtins/tracing.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, ensure_string};
+use crate::lexer::Span;
+use crate::value::Value;
+
+use std::collections::HashMap;
+
+use anyhow::Result;
+
+pub fn register(m: &mut HashMap<&'static str, builtins::BuiltinFcn>) {
+    m.insert("trace", trace);
+}
+
+// Symbol analyzer must ensure that vars used by trace are defined before
+// the trace statement. Scheduler must ensure the above constraint.
+fn trace(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
+    let name = "trace";
+    ensure_args_count(span, name, params, args, 1)?;
+    let msg = ensure_string(name, &params[0], &args[0])?;
+
+    // Unlike rego, trace returns a string instead of bool.
+    // The interpreter accumulates the traces.
+    // TODO: Stateful bultins can pass in a state that would allow capturing
+    // the traces in the state.
+    Ok(Value::String(msg))
+}