diff --git a/scripts/yaml-test-eval b/scripts/yaml-test-eval
index 7bc185c2..5f6f9435 100755
--- a/scripts/yaml-test-eval
+++ b/scripts/yaml-test-eval
@@ -5,4 +5,4 @@
 set -e
 yaml=$(realpath -e $1)
 
-RUST_BACKTRACE=1 cargo test interpreter::one_yaml -- --include-ignored --nocapture "$yaml"
+RUST_BACKTRACE=1 cargo test interpreter::one_yaml -- --include-ignored --nocapture "$yaml" $2
diff --git a/src/builtins/aggregates.rs b/src/builtins/aggregates.rs
index 65422dac..98e9ab07 100644
--- a/src/builtins/aggregates.rs
+++ b/src/builtins/aggregates.rs
@@ -12,12 +12,12 @@ use std::collections::HashMap;
 use anyhow::{bail, Result};
 
 pub fn register(m: &mut HashMap<&'static str, builtins::BuiltinFcn>) {
-    m.insert("count", count);
-    m.insert("max", max);
-    m.insert("min", min);
-    m.insert("product", product);
-    m.insert("sort", sort);
-    m.insert("sum", sum);
+    m.insert("count", (count, 1));
+    m.insert("max", (max, 1));
+    m.insert("min", (min, 1));
+    m.insert("product", (product, 1));
+    m.insert("sort", (sort, 1));
+    m.insert("sum", (sum, 1));
 }
 
 fn count(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
@@ -57,9 +57,9 @@ fn min(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
 
     Ok(match &args[0] {
         Value::Array(a) if a.is_empty() => Value::Undefined,
-        Value::Array(a) => a.iter().max().unwrap().clone(),
+        Value::Array(a) => a.iter().min().unwrap().clone(),
         Value::Set(a) if a.is_empty() => Value::Undefined,
-        Value::Set(a) => a.iter().max().unwrap().clone(),
+        Value::Set(a) => a.iter().min().unwrap().clone(),
         a => {
             let span = params[0].span();
             bail!(span.error(format!("`min` requires array/set argument. Got `{a}`.").as_str()))
@@ -68,7 +68,7 @@ fn min(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
 }
 
 fn product(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
-    ensure_args_count(span, "min", params, args, 1)?;
+    ensure_args_count(span, "product", params, args, 1)?;
 
     let mut v = 1 as Float;
     Ok(match &args[0] {
diff --git a/src/builtins/arrays.rs b/src/builtins/arrays.rs
index 4984a891..d9a8294b 100644
--- a/src/builtins/arrays.rs
+++ b/src/builtins/arrays.rs
@@ -13,9 +13,9 @@ use anyhow::Result;
 use std::rc::Rc;
 
 pub fn register(m: &mut HashMap<&'static str, builtins::BuiltinFcn>) {
-    m.insert("array.concat", concat);
-    m.insert("array.reverse", reverse);
-    m.insert("array.slice", slice);
+    m.insert("array.concat", (concat, 2));
+    m.insert("array.reverse", (reverse, 1));
+    m.insert("array.slice", (slice, 3));
 }
 
 fn concat(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
diff --git a/src/builtins/bitwise.rs b/src/builtins/bitwise.rs
index ef1efcf9..c866e404 100644
--- a/src/builtins/bitwise.rs
+++ b/src/builtins/bitwise.rs
@@ -13,12 +13,12 @@ use std::collections::HashMap;
 use anyhow::Result;
 
 pub fn register(m: &mut HashMap<&'static str, builtins::BuiltinFcn>) {
-    m.insert("bits.and", and);
-    m.insert("bits.lsh", lsh);
-    m.insert("bits.negate", negate);
-    m.insert("bits.or", or);
-    m.insert("bits.rsh", rsh);
-    m.insert("bits.xor", xor);
+    m.insert("bits.and", (and, 2));
+    m.insert("bits.lsh", (lsh, 2));
+    m.insert("bits.negate", (negate, 1));
+    m.insert("bits.or", (or, 2));
+    m.insert("bits.rsh", (rsh, 2));
+    m.insert("bits.xor", (xor, 2));
 }
 
 fn and(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
diff --git a/src/builtins/conversions.rs b/src/builtins/conversions.rs
index 58e936c3..88aac5fd 100644
--- a/src/builtins/conversions.rs
+++ b/src/builtins/conversions.rs
@@ -12,7 +12,7 @@ use std::collections::HashMap;
 use anyhow::{bail, Result};
 
 pub fn register(m: &mut HashMap<&'static str, builtins::BuiltinFcn>) {
-    m.insert("to_number", to_number);
+    m.insert("to_number", (to_number, 1));
 }
 
 fn to_number(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
diff --git a/src/builtins/debugging.rs b/src/builtins/debugging.rs
index 34d06243..3c76fcfc 100644
--- a/src/builtins/debugging.rs
+++ b/src/builtins/debugging.rs
@@ -8,16 +8,23 @@ use crate::value::Value;
 
 use std::collections::HashMap;
 
-use anyhow::Result;
+use anyhow::{bail, Result};
+
+// TODO: Should we avoid this limit?
+const MAX_ARGS: u8 = std::u8::MAX;
 
 pub fn register(m: &mut HashMap<&'static str, builtins::BuiltinFcn>) {
-    m.insert("print", print);
+    m.insert("print", (print, MAX_ARGS));
 }
 
 // Symbol analyzer must ensure that vars used by print are defined before
 // the print statement. Scheduler must ensure the above constraint.
 // Additionally interpreter must allow undefined inputs to print.
 fn print(span: &Span, _params: &[Expr], args: &[Value]) -> Result<Value> {
+    if args.len() > MAX_ARGS as usize {
+        bail!(span.error("print supports up to 100 arguments"));
+    }
+
     let mut msg = String::default();
     for a in args {
         match a {
diff --git a/src/builtins/mod.rs b/src/builtins/mod.rs
index e3289700..b040aad4 100644
--- a/src/builtins/mod.rs
+++ b/src/builtins/mod.rs
@@ -24,7 +24,7 @@ use std::collections::HashMap;
 use anyhow::Result;
 use lazy_static::lazy_static;
 
-pub type BuiltinFcn = fn(&Span, &[Expr], &[Value]) -> Result<Value>;
+pub type BuiltinFcn = (fn(&Span, &[Expr], &[Value]) -> Result<Value>, u8);
 
 #[rustfmt::skip]
 lazy_static! {
diff --git a/src/builtins/numbers.rs b/src/builtins/numbers.rs
index 08eff660..7f65c770 100644
--- a/src/builtins/numbers.rs
+++ b/src/builtins/numbers.rs
@@ -13,12 +13,12 @@ use anyhow::Result;
 use rand::{thread_rng, Rng};
 
 pub fn register(m: &mut HashMap<&'static str, builtins::BuiltinFcn>) {
-    m.insert("abs", abs);
-    m.insert("ceil", ceil);
-    m.insert("floor", floor);
-    m.insert("numbers.range", range);
-    m.insert("rand.intn", intn);
-    m.insert("round", round);
+    m.insert("abs", (abs, 1));
+    m.insert("ceil", (ceil, 1));
+    m.insert("floor", (floor, 1));
+    m.insert("numbers.range", (range, 2));
+    m.insert("rand.intn", (intn, 2));
+    m.insert("round", (round, 1));
 }
 
 pub fn arithmetic_operation(
diff --git a/src/builtins/objects.rs b/src/builtins/objects.rs
index 3bb89669..f88aadcc 100644
--- a/src/builtins/objects.rs
+++ b/src/builtins/objects.rs
@@ -14,12 +14,12 @@ use std::rc::Rc;
 use anyhow::{bail, Result};
 
 pub fn register(m: &mut HashMap<&'static str, builtins::BuiltinFcn>) {
-    m.insert("json.filter", json_filter);
-    //    m.insert("json.patch", json_patch);
-    m.insert("object.filter", filter);
-    m.insert("object.get", get);
-    m.insert("object.keys", keys);
-    m.insert("object.remove", remove);
+    m.insert("json.filter", (json_filter, 2));
+    //    m.insert("json.patch", (json_patch));
+    m.insert("object.filter", (filter, 2));
+    m.insert("object.get", (get, 3));
+    m.insert("object.keys", (keys, 1));
+    m.insert("object.remove", (remove, 2));
 }
 
 fn json_filter_impl(v: &Value, filter: &Value) -> Value {
diff --git a/src/builtins/sets.rs b/src/builtins/sets.rs
index de5e65be..d62fd448 100644
--- a/src/builtins/sets.rs
+++ b/src/builtins/sets.rs
@@ -12,8 +12,8 @@ use std::collections::{BTreeSet, HashMap};
 use anyhow::{bail, Result};
 
 pub fn register(m: &mut HashMap<&'static str, builtins::BuiltinFcn>) {
-    m.insert("intersection", intersection_of_set_of_sets);
-    m.insert("union", union_of_set_of_sets);
+    m.insert("intersection", (intersection_of_set_of_sets, 1));
+    m.insert("union", (union_of_set_of_sets, 1));
 }
 
 pub fn intersection(expr1: &Expr, expr2: &Expr, v1: Value, v2: Value) -> Result<Value> {
diff --git a/src/builtins/strings.rs b/src/builtins/strings.rs
index 446698bb..0392768c 100644
--- a/src/builtins/strings.rs
+++ b/src/builtins/strings.rs
@@ -15,29 +15,29 @@ use std::collections::HashMap;
 use anyhow::{bail, Result};
 
 pub fn register(m: &mut HashMap<&'static str, builtins::BuiltinFcn>) {
-    m.insert("concat", concat);
-    m.insert("contains", contains);
-    m.insert("endswith", endswith);
-    m.insert("format_int", format_int);
-    m.insert("indexof", indexof);
-    m.insert("indexof_n", indexof_n);
-    m.insert("lower", lower);
-    m.insert("replace", replace);
-    m.insert("split", split);
-    m.insert("sprintf", sprintf);
-    m.insert("startswith", startswith);
-    m.insert("strings.any_prefix_match", any_prefix_match);
-    m.insert("strings.any_suffix_match", any_suffix_match);
-    m.insert("strings.replace_n", replace_n);
-    m.insert("strings.reverse", reverse);
-    m.insert("strings.substring", substring);
-    m.insert("trim", trim);
-    m.insert("trim_left", trim_left);
-    m.insert("trim_prefix", trim_prefix);
-    m.insert("trim_right", trim_right);
-    m.insert("trim_space", trim_space);
-    m.insert("trim_suffix", trim_suffix);
-    m.insert("upper", upper);
+    m.insert("concat", (concat, 2));
+    m.insert("contains", (contains, 2));
+    m.insert("endswith", (endswith, 2));
+    m.insert("format_int", (format_int, 2));
+    m.insert("indexof", (indexof, 2));
+    m.insert("indexof_n", (indexof_n, 2));
+    m.insert("lower", (lower, 1));
+    m.insert("replace", (replace, 3));
+    m.insert("split", (split, 2));
+    m.insert("sprintf", (sprintf, 2));
+    m.insert("startswith", (startswith, 2));
+    m.insert("strings.any_prefix_match", (any_prefix_match, 2));
+    m.insert("strings.any_suffix_match", (any_suffix_match, 2));
+    m.insert("strings.replace_n", (replace_n, 2));
+    m.insert("strings.reverse", (reverse, 1));
+    m.insert("strings.substring", (substring, 3));
+    m.insert("trim", (trim, 2));
+    m.insert("trim_left", (trim_left, 2));
+    m.insert("trim_prefix", (trim_prefix, 2));
+    m.insert("trim_right", (trim_right, 2));
+    m.insert("trim_space", (trim_space, 1));
+    m.insert("trim_suffix", (trim_suffix, 2));
+    m.insert("upper", (upper, 1));
 }
 
 fn concat(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
diff --git a/src/builtins/tracing.rs b/src/builtins/tracing.rs
index 7f68fb2b..d63cc302 100644
--- a/src/builtins/tracing.rs
+++ b/src/builtins/tracing.rs
@@ -12,7 +12,7 @@ use std::collections::HashMap;
 use anyhow::Result;
 
 pub fn register(m: &mut HashMap<&'static str, builtins::BuiltinFcn>) {
-    m.insert("trace", trace);
+    m.insert("trace", (trace, 1));
 }
 
 // Symbol analyzer must ensure that vars used by trace are defined before
diff --git a/src/builtins/types.rs b/src/builtins/types.rs
index 20a79ff3..9b70f827 100644
--- a/src/builtins/types.rs
+++ b/src/builtins/types.rs
@@ -12,14 +12,14 @@ use std::collections::HashMap;
 use anyhow::Result;
 
 pub fn register(m: &mut HashMap<&'static str, builtins::BuiltinFcn>) {
-    m.insert("is_array", is_array);
-    m.insert("is_boolean", is_boolean);
-    m.insert("is_null", is_null);
-    m.insert("is_number", is_number);
-    m.insert("is_object", is_object);
-    m.insert("is_set", is_set);
-    m.insert("is_string", is_string);
-    m.insert("type_name", type_name);
+    m.insert("is_array", (is_array, 1));
+    m.insert("is_boolean", (is_boolean, 1));
+    m.insert("is_null", (is_null, 1));
+    m.insert("is_number", (is_number, 1));
+    m.insert("is_object", (is_object, 1));
+    m.insert("is_set", (is_set, 1));
+    m.insert("is_string", (is_string, 1));
+    m.insert("type_name", (type_name, 1));
 }
 
 fn is_array(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
diff --git a/src/interpreter.rs b/src/interpreter.rs
index 64497c6a..236773fb 100644
--- a/src/interpreter.rs
+++ b/src/interpreter.rs
@@ -6,6 +6,7 @@ use crate::builtins;
 use crate::lexer::Span;
 use crate::parser::Parser;
 use crate::scheduler::*;
+use crate::utils::*;
 use crate::value::*;
 
 use anyhow::{anyhow, bail, Result};
@@ -164,6 +165,8 @@ impl<'source> Interpreter<'source> {
         if let Some(variable) = self.current_scope_mut()?.get_mut(name) {
             *variable = value.clone();
             Ok(())
+        } else if name == "_" {
+            Ok(())
         } else {
             bail!("variable {} is undefined", name)
         }
@@ -783,7 +786,17 @@ impl<'source> Interpreter<'source> {
 
         let r = Ok(match &stmt.literal {
             Literal::Expr { expr, .. } => {
-                let value = self.eval_expr(expr)?;
+                let value = match expr {
+                    Expr::Call { span, fcn, params } => self.eval_call(
+                        span,
+                        fcn,
+                        params,
+                        get_extra_arg(expr, &HashMap::new()),
+                        true,
+                    )?,
+                    _ => self.eval_expr(expr)?,
+                };
+
                 if let Value::Bool(bool) = value {
                     bool
                 } else {
@@ -793,6 +806,21 @@ impl<'source> Interpreter<'source> {
                     value != Value::Undefined
                 }
             }
+            Literal::NotExpr { expr, .. } => {
+                let value = match expr {
+                    // Extra parameter is allowed; but a return argument is not allowed.
+                    Expr::Call { span, fcn, params } => self.eval_call(
+                        span,
+                        fcn,
+                        params,
+                        get_extra_arg(expr, &HashMap::new()),
+                        false,
+                    )?,
+                    _ => self.eval_expr(expr)?,
+                };
+                // https://github.com/open-policy-agent/opa/issues/1622#issuecomment-520547385
+                matches!(value, Value::Bool(false) | Value::Undefined)
+            }
             Literal::SomeVars { vars, .. } => {
                 for var in vars {
                     let name = var.text();
@@ -813,10 +841,6 @@ impl<'source> Interpreter<'source> {
                 value,
                 collection,
             } => self.eval_some_in(span, key, value, collection, stmts)?,
-            Literal::NotExpr { expr, .. } => {
-                // https://github.com/open-policy-agent/opa/issues/1622#issuecomment-520547385
-                matches!(self.eval_expr(expr)?, Value::Bool(false) | Value::Undefined)
-            }
             Literal::Every {
                 span,
                 key,
@@ -1278,7 +1302,7 @@ impl<'source> Interpreter<'source> {
         span: &'source Span<'source>,
         name: String,
         builtin: builtins::BuiltinFcn,
-        params: &'source Vec<Expr<'source>>,
+        params: &'source [Expr<'source>],
     ) -> Result<Value> {
         let mut args = vec![];
         let allow_undefined = name == "print"; // TODO: with modifier
@@ -1297,7 +1321,7 @@ impl<'source> Interpreter<'source> {
             }
         }
 
-        let v = builtin(span, &params[..], &args[..])?;
+        let v = builtin.0(span, params, &args[..])?;
 
         // Handle trace function.
         // TODO: with modifier.
@@ -1312,11 +1336,11 @@ impl<'source> Interpreter<'source> {
         Ok(v)
     }
 
-    fn eval_call(
+    fn eval_call_impl(
         &mut self,
         span: &'source Span<'source>,
         fcn: &'source Expr<'source>,
-        params: &'source Vec<Expr<'source>>,
+        params: &'source [Expr<'source>],
     ) -> Result<Value> {
         let fcns_rules = match self.lookup_function(fcn) {
             Ok(r) => r,
@@ -1438,6 +1462,34 @@ impl<'source> Interpreter<'source> {
         Ok(results[0].clone())
     }
 
+    fn eval_call(
+        &mut self,
+        span: &'source Span<'source>,
+        fcn: &'source Expr<'source>,
+        params: &'source Vec<Expr<'source>>,
+        extra_arg: Option<&'source Expr<'source>>,
+        allow_return_arg: bool,
+    ) -> Result<Value> {
+        // TODO: global var check; interop with `some var`
+        match extra_arg {
+            Some(Expr::Var(var))
+                if allow_return_arg && self.lookup_local_var(var.text()).is_none() =>
+            {
+                let value = self.eval_call_impl(span, fcn, &params[..params.len() - 1])?;
+                if var.text() != "_" {
+                    self.add_variable(var.text(), value)?;
+                }
+                Ok(Value::Bool(true))
+            }
+            Some(expr) => {
+                let ret_value = self.eval_call_impl(span, fcn, &params[..params.len() - 1])?;
+                let value = self.eval_expr(expr)?;
+                Ok(Value::Bool(ret_value == value))
+            }
+            None => self.eval_call_impl(span, fcn, params),
+        }
+    }
+
     fn lookup_local_var(&self, name: &str) -> Option<Value> {
         // Lookup local variables and arguments.
         for scope in self.scopes.iter().rev() {
@@ -1566,7 +1618,7 @@ impl<'source> Interpreter<'source> {
             } => self.eval_object_compr(key, value, query),
             Expr::SetCompr { term, query, .. } => self.eval_set_compr(term, query),
             Expr::UnaryExpr { .. } => unimplemented!("unar expr is umplemented"),
-            Expr::Call { span, fcn, params } => self.eval_call(span, fcn, params),
+            Expr::Call { span, fcn, params } => self.eval_call(span, fcn, params, None, false),
         }
     }
 
@@ -2150,8 +2202,21 @@ impl<'source> Interpreter<'source> {
         let prev_module = self.set_current_module(self.modules.last().copied())?;
         let value = self.eval_expr(snippet)?;
         // Pop the scope.
+
         let scope = self.scopes.pop();
-        let r = match snippet {
+        let r = match scope {
+            Some(scope) if !scope.is_empty() => {
+                let mut r = Value::new_object();
+                let map = r.as_object_mut()?;
+                // Capture each binding.
+                for (name, v) in scope {
+                    map.insert(Value::String(name), v);
+                }
+                Ok(r)
+            }
+            _ => Ok(value),
+        };
+        /*        let r = match snippet {
             Expr::AssignExpr { .. } => {
                 if let Some(scope) = scope {
                     let mut r = Value::new_object();
@@ -2166,7 +2231,7 @@ impl<'source> Interpreter<'source> {
                 }
             }
             _ => Ok(value),
-        };
+        };*/
         self.set_current_module(prev_module)?;
         r
     }
diff --git a/src/lib.rs b/src/lib.rs
index a86cc639..f3787594 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -7,6 +7,7 @@ pub mod interpreter;
 pub mod lexer;
 pub mod parser;
 pub mod scheduler;
+mod utils;
 pub mod value;
 
 pub use ast::*;
diff --git a/src/parser.rs b/src/parser.rs
index 8e127c37..9fecddc5 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -738,7 +738,7 @@ impl<'source> Parser<'source> {
         Ok(expr)
     }
 
-    fn parse_assign_expr(&mut self) -> Result<Expr<'source>> {
+    pub fn parse_assign_expr(&mut self) -> Result<Expr<'source>> {
         let state = self.clone();
         let start = self.tok.1.start;
         let expr = self.parse_ref()?;
diff --git a/src/scheduler.rs b/src/scheduler.rs
index 1415b194..d8340d2e 100644
--- a/src/scheduler.rs
+++ b/src/scheduler.rs
@@ -3,10 +3,10 @@
 
 use crate::ast::Expr::*;
 use crate::ast::*;
-use crate::interpreter::Interpreter;
 use crate::lexer::Span;
+use crate::utils;
 
-use std::collections::{BTreeMap, BTreeSet, VecDeque};
+use std::collections::{BTreeMap, BTreeSet, HashMap, VecDeque};
 use std::string::String;
 
 use anyhow::{bail, Result};
@@ -187,7 +187,7 @@ pub fn schedule<'a>(infos: &mut [StmtInfo<'a>]) -> Result<SortResult> {
     }
 
     if order.len() != num_statements {
-        bail!("could not schedule all statements {order:?} {num_statements}");
+        bail!("could not schedule all statements {order:?} {num_statements} {tmp:?}");
     }
 
     // TODO: determine cycles.
@@ -405,7 +405,7 @@ impl<'a> Analyzer<'a> {
 
     pub fn analyze(mut self, modules: &'a [Module<'a>]) -> Result<Schedule> {
         for m in modules {
-            let path = Interpreter::get_path_string(&m.package.refr, Some("data"))?;
+            let path = utils::get_path_string(&m.package.refr, Some("data"))?;
             let scope: &mut Scope = self.packages.entry(path).or_default();
             for r in &m.policy {
                 let var = match r {
@@ -433,7 +433,7 @@ impl<'a> Analyzer<'a> {
     }
 
     fn analyze_module(&mut self, m: &'a Module<'a>) -> Result<()> {
-        let path = Interpreter::get_path_string(&m.package.refr, Some("data"))?;
+        let path = utils::get_path_string(&m.package.refr, Some("data"))?;
         let scope = match self.packages.get(&path) {
             Some(s) => s,
             _ => bail!("internal error: package scope missing"),
@@ -932,7 +932,36 @@ impl<'a> Analyzer<'a> {
                     definitions.push(Definition { var: "", used_vars });
                     // TODO: vars in compr
                 }
-                Literal::Expr { expr, .. } | Literal::NotExpr { expr, .. } => {
+                Literal::Expr { expr, .. } => {
+                    if let Some(Expr::Var(return_arg)) = utils::get_extra_arg(expr, &HashMap::new())
+                    {
+                        let (mut used_vars, comprs) = Self::gather_used_vars_comprs_index_vars(
+                            expr,
+                            &mut scope,
+                            &mut first_use,
+                            &mut definitions,
+                        )?;
+                        let var = if return_arg.text() != "_" {
+                            // The var in the return argument slot would have been processed as
+                            // an used var. Remove it from used vars and add it as the variable being
+                            // defined.
+                            used_vars.pop();
+                            return_arg.text()
+                        } else {
+                            ""
+                        };
+                        self.process_comprs(
+                            &comprs[..],
+                            &mut scope,
+                            &mut first_use,
+                            &mut used_vars,
+                        )?;
+                        definitions.push(Definition { var, used_vars });
+                    } else {
+                        self.process_expr(expr, &mut scope, &mut first_use, &mut definitions)?;
+                    }
+                }
+                Literal::NotExpr { expr, .. } => {
                     self.process_expr(expr, &mut scope, &mut first_use, &mut definitions)?;
                 }
                 Literal::Every {
diff --git a/src/utils.rs b/src/utils.rs
new file mode 100644
index 00000000..a554d4ee
--- /dev/null
+++ b/src/utils.rs
@@ -0,0 +1,59 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+use crate::ast::*;
+use crate::builtins::*;
+
+use std::collections::HashMap;
+
+use anyhow::{bail, Result};
+
+pub fn get_path_string(refr: &Expr, document: Option<&str>) -> Result<String> {
+    let mut comps = vec![];
+    let mut expr = Some(refr);
+    while expr.is_some() {
+        match expr {
+            Some(Expr::RefDot { refr, field, .. }) => {
+                comps.push(field.text());
+                expr = Some(refr);
+            }
+            Some(Expr::RefBrack { refr, index, .. })
+                if matches!(index.as_ref(), Expr::String(_)) =>
+            {
+                if let Expr::String(s) = index.as_ref() {
+                    comps.push(s.text());
+                    expr = Some(refr);
+                }
+            }
+            Some(Expr::Var(v)) => {
+                comps.push(v.text());
+                expr = None;
+            }
+            _ => bail!("internal error: not a simple ref"),
+        }
+    }
+    if let Some(d) = document {
+        comps.push(d);
+    };
+    comps.reverse();
+    Ok(comps.join("."))
+}
+
+pub fn get_extra_arg<'a>(expr: &'a Expr, arities: &HashMap<String, u8>) -> Option<&'a Expr<'a>> {
+    if let Expr::Call { fcn, params, .. } = expr {
+        if let Ok(path) = get_path_string(fcn, None) {
+            let n_args = if let Some(n_args) = arities.get(&path) {
+                *n_args
+            } else if let Some((_, n_args)) = BUILTINS.get(path.as_str()) {
+                *n_args
+            } else {
+                return None;
+            };
+            if n_args as usize == params.len() - 1 {
+                return params.last();
+            }
+        }
+    }
+
+    None
+}
diff --git a/tests/interpreter/cases/builtins/aggregates/min.yaml b/tests/interpreter/cases/builtins/aggregates/min.yaml
index 20a657d5..01557612 100644
--- a/tests/interpreter/cases/builtins/aggregates/min.yaml
+++ b/tests/interpreter/cases/builtins/aggregates/min.yaml
@@ -19,7 +19,7 @@ cases:
         u3 = min(set())
     query: data.test
     want_result:
-      x: [ 3, 2 ]
+      x: [ -1, -1 ]
 
   - note: invalid-null
     data: {}
diff --git a/tests/interpreter/cases/call/oldstyle.yaml b/tests/interpreter/cases/call/oldstyle.yaml
new file mode 100644
index 00000000..a7a998af
--- /dev/null
+++ b/tests/interpreter/cases/call/oldstyle.yaml
@@ -0,0 +1,19 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+cases:
+  - note: basic
+    data: {}
+    modules:
+      - |
+        package test
+
+        a = [1, 2, 3, 4]
+
+        p = x {
+          count(a, x)
+        }
+
+    query: data.test.p = x
+    want_result:
+      x: 4
diff --git a/tests/interpreter/cases/compr/mod.rs b/tests/interpreter/cases/compr/mod.rs
index f7cadbc5..6e0ba091 100644
--- a/tests/interpreter/cases/compr/mod.rs
+++ b/tests/interpreter/cases/compr/mod.rs
@@ -32,7 +32,7 @@ fn basic_array() -> Result<()> {
     array_compr_7 = [ 1 | [1, 2, 3][_]; [1, 2][_] >= 2 ]
 "#;
 
-    let expected = vec![Value::from_json_str(
+    let expected = [Value::from_json_str(
         r#" {
             "array": [1, 2, 3],
             "array_compr_0": [1],
@@ -46,11 +46,10 @@ fn basic_array() -> Result<()> {
 }"#,
     )?];
 
-    assert_match(
-        eval_file(&[rego.to_owned()], None, None, "data.test", false)?,
-        expected,
-    );
-    Ok(())
+    check_output(
+        &eval_file(&[rego.to_owned()], None, None, "data.test", false)?,
+        &expected,
+    )
 }
 
 #[test]
@@ -81,7 +80,7 @@ fn basic_set() -> Result<()> {
     set_compr_7 = { a | a = [1, 2, 3][_]; [1, 2][_] >= 2 }
 "#;
 
-    let expected = vec![Value::from_json_str(
+    let expected = [Value::from_json_str(
         r#" {
             "set": {
                 "set!": [1, "string", [2, 3, 4], 567, false]
@@ -115,9 +114,8 @@ fn basic_set() -> Result<()> {
  }"#,
     )?];
 
-    assert_match(
-        eval_file(&[rego.to_owned()], None, None, "data.test", false)?,
-        expected,
-    );
-    Ok(())
+    check_output(
+        &eval_file(&[rego.to_owned()], None, None, "data.test", false)?,
+        &expected,
+    )
 }
diff --git a/tests/interpreter/cases/in/mod.rs b/tests/interpreter/cases/in/mod.rs
index 1ac45aea..7be01a87 100644
--- a/tests/interpreter/cases/in/mod.rs
+++ b/tests/interpreter/cases/in/mod.rs
@@ -99,7 +99,7 @@ fn basic() -> Result<()> {
     }
 "#;
 
-    let expected = vec![Value::from_json_str(
+    let expected = [Value::from_json_str(
         r#" {
             "array": [1, 2, 3],
             "in_array_key_value": true,
@@ -119,9 +119,8 @@ fn basic() -> Result<()> {
 }"#,
     )?];
 
-    assert_match(
-        eval_file(&[rego.to_owned()], None, None, "data.test", false)?,
-        expected,
-    );
-    Ok(())
+    check_output(
+        &eval_file(&[rego.to_owned()], None, None, "data.test", false)?,
+        &expected,
+    )
 }
diff --git a/tests/interpreter/cases/input/mod.rs b/tests/interpreter/cases/input/mod.rs
index f1e6943f..c8efccfb 100644
--- a/tests/interpreter/cases/input/mod.rs
+++ b/tests/interpreter/cases/input/mod.rs
@@ -25,7 +25,7 @@ fn basic() -> Result<()> {
         Value::from_json_str(r#"{"x": 6}"#)?,
     ]);
 
-    let expected = vec![
+    let expected = [
         Value::from_json_str(
             r#" {
         "y": {"set!": [6]},
@@ -40,9 +40,8 @@ fn basic() -> Result<()> {
         )?,
     ];
 
-    assert_match(
-        eval_file_first_rule(&[rego.to_owned()], None, Some(input), "data.test", false)?,
-        expected,
-    );
-    Ok(())
+    check_output(
+        &eval_file_first_rule(&[rego.to_owned()], None, Some(input), "data.test", false)?,
+        &expected,
+    )
 }
diff --git a/tests/interpreter/cases/variables/mod.rs b/tests/interpreter/cases/variables/mod.rs
index 592d9a3f..35d34acf 100644
--- a/tests/interpreter/cases/variables/mod.rs
+++ b/tests/interpreter/cases/variables/mod.rs
@@ -31,7 +31,7 @@ fn basic() -> Result<()> {
     set = {1, 2, 3}
 "#;
 
-    let expected = vec![Value::from_json_str(
+    let expected = [Value::from_json_str(
         r#" {
     "array": [1, 2, 3],
     "nested_array": [1, [2, 3, 4], 5, 6],
@@ -46,9 +46,8 @@ fn basic() -> Result<()> {
 }"#,
     )?];
 
-    assert_match(
-        eval_file(&[rego.to_owned()], None, None, "data.test", false)?,
-        expected,
-    );
-    Ok(())
+    check_output(
+        &eval_file(&[rego.to_owned()], None, None, "data.test", false)?,
+        &expected,
+    )
 }
diff --git a/tests/interpreter/mod.rs b/tests/interpreter/mod.rs
index 8638c138..72201ac9 100644
--- a/tests/interpreter/mod.rs
+++ b/tests/interpreter/mod.rs
@@ -4,12 +4,13 @@
 #![cfg(test)]
 
 use std::env;
+use std::path::Path;
 
 use anyhow::{bail, Result};
 use regorus::*;
 use serde::{ser::SerializeMap, Deserialize, Deserializer, Serialize, Serializer};
 use test_generator::test_resources;
-//use walkdir::WalkDir;
+use walkdir::WalkDir;
 
 mod cases;
 
@@ -154,28 +155,30 @@ fn match_values(computed: &Value, expected: &Value) -> Result<()> {
     }
 }
 
-pub fn assert_match(computed_results: Vec<Value>, expected_results: Vec<Value>) {
+pub fn check_output(computed_results: &[Value], expected_results: &[Value]) -> Result<()> {
     if computed_results.len() != expected_results.len() {
-        panic!(
+        bail!(
             "the number of computed results ({}) and expected results ({}) is not equal",
             computed_results.len(),
             expected_results.len()
         );
     }
 
-    for (n, expected_result) in expected_results.into_iter().enumerate() {
-        let expected = match process_value(&expected_result) {
+    for (n, expected_result) in expected_results.iter().enumerate() {
+        let expected = match process_value(expected_result) {
             Ok(e) => e,
-            _ => panic!("unable to process value :\n {expected_result:?}"),
+            _ => bail!("unable to process value :\n {expected_result:?}"),
         };
 
         if let Some(computed_result) = computed_results.get(n) {
             match match_values(computed_result, &expected) {
                 Ok(()) => (),
-                Err(e) => panic!("{}", e),
+                Err(e) => bail!("{e}"),
             }
         }
     }
+
+    Ok(())
 }
 
 pub fn eval_file_first_rule(
@@ -198,7 +201,7 @@ pub fn eval_file_first_rule(
         lines: query.split('\n').collect(),
     };
     let mut parser = Parser::new(&source)?;
-    let expr = parser.parse_membership_expr()?;
+    let expr = parser.parse_assign_expr()?;
 
     for (idx, _) in regos.iter().enumerate() {
         files.push(format!("rego_{idx}"));
@@ -278,7 +281,7 @@ pub fn eval_file(
         lines: query.split('\n').collect(),
     };
     let mut parser = Parser::new(&source)?;
-    let expr = parser.parse_membership_expr()?;
+    let expr = parser.parse_assign_expr()?;
 
     for (idx, _) in regos.iter().enumerate() {
         files.push(format!("rego_{idx}"));
@@ -435,7 +438,7 @@ struct YamlTest {
     cases: Vec<TestCase>,
 }
 
-fn yaml_test_impl(file: &str) -> Result<()> {
+fn yaml_test_impl(file: &str, is_opa_test: bool) -> Result<()> {
     let yaml_str = std::fs::read_to_string(file)?;
     let test: YamlTest = serde_yaml::from_str(&yaml_str)?;
 
@@ -472,7 +475,14 @@ fn yaml_test_impl(file: &str) -> Result<()> {
                         }
                     }
 
-                    assert_match(results, expected_results);
+                    if is_opa_test {
+                        // Convert value to json compatible representation.
+                        let results =
+                            Value::from_json_str(serde_json::to_string(&results)?.as_str())?;
+                        match_values(&results, &expected_results[0])?;
+                    } else {
+                        check_output(&results, &expected_results)?;
+                    }
                 }
                 _ => panic!("eval succeeded and did not produce any errors"),
             },
@@ -498,8 +508,8 @@ fn yaml_test_impl(file: &str) -> Result<()> {
     Ok(())
 }
 
-fn yaml_test(file: &str) -> Result<()> {
-    match yaml_test_impl(file) {
+fn yaml_test(file: &str, is_opa_test: bool) -> Result<()> {
+    match yaml_test_impl(file, is_opa_test) {
         Ok(_) => Ok(()),
         Err(e) => {
             // If Err is returned, it doesn't always get printed by cargo test.
@@ -511,28 +521,65 @@ fn yaml_test(file: &str) -> Result<()> {
 
 #[test]
 fn yaml_test_basic() -> Result<()> {
-    yaml_test("tests/interpreter/cases/basic_001.yaml")
+    yaml_test("tests/interpreter/cases/basic_001.yaml", false)
 }
 
 #[test]
 #[ignore = "intended for use by scripts/yaml-test-eval"]
 fn one_yaml() -> Result<()> {
     let mut file = String::default();
+    let mut is_opa_test = false;
+
     for a in env::args() {
         if a.ends_with(".yaml") {
             file = a;
-            break;
+        } else if a == "opa-test" {
+            is_opa_test = true;
         }
     }
 
     if file.is_empty() {
-        bail!("missing <policy.rego>");
+        bail!("missing <yaml-file>");
     }
 
-    yaml_test(file.as_str())
+    yaml_test(file.as_str(), is_opa_test)
 }
 
 #[test_resources("tests/interpreter/**/*.yaml")]
 fn run(path: &str) {
-    yaml_test(path).unwrap()
+    yaml_test(path, false).unwrap()
+}
+
+#[test]
+#[ignore = "intended for running opa test suite"]
+fn run_opa_tests() -> Result<()> {
+    let mut failures = vec![];
+    for a in env::args() {
+        if !Path::new(&a).is_dir() {
+            continue;
+        }
+        for entry in WalkDir::new(a)
+            .sort_by_file_name()
+            .into_iter()
+            .filter_map(|e| e.ok())
+        {
+            let path = entry.path().to_string_lossy().to_string();
+            if Path::new(&path).is_dir() {
+                continue;
+            }
+            let yaml = path;
+            match yaml_test_impl(yaml.as_str(), true) {
+                Ok(_) => (),
+                Err(e) => {
+                    failures.push((yaml, e));
+                }
+            }
+        }
+    }
+
+    if !failures.is_empty() {
+        dbg!(failures);
+        panic!("failed");
+    }
+    Ok(())
 }