feat: Format strings for prints

crates/nargo/Cargo.toml

@@ -20,4 +20,5 @@ serde.workspace = true
 serde_json.workspace = true
 thiserror.workspace = true
 noirc_errors.workspace = true
-base64.workspace = true
+base64.workspace = true
+regex = "1.9.1"

crates/nargo/src/ops/foreign_calls.rs

@@ -4,6 +4,7 @@ use acvm::{
 };
 use iter_extended::vecmap;
 use noirc_abi::{decode_string_value, input_parser::InputValueDisplay, AbiType};
+use regex::{Captures, Regex};
 
 use crate::errors::ForeignCallError;
 
@@ -63,31 +64,89 @@ impl ForeignCall {
     }
 
     fn execute_println(foreign_call_inputs: &[Vec<Value>]) -> Result<(), ForeignCallError> {
-        let (abi_type, input_values) = fetch_abi_type(foreign_call_inputs)?;
+        let (is_fmt_str, foreign_call_inputs) =
+            foreign_call_inputs.split_last().ok_or(ForeignCallError::MissingForeignCallInputs)?;
 
-        // We must use a flat map here as each value in a struct will be in a separate input value
-        let mut input_values_as_fields =
-            input_values.iter().flat_map(|values| values.iter().map(|value| value.to_field()));
-
-        let input_value_display =
-            InputValueDisplay::try_from_fields(&mut input_values_as_fields, abi_type)?;
-
-        println!("{input_value_display}");
+        let output_string = if is_fmt_str[0].to_field().is_one() {
+            convert_fmt_string_inputs(foreign_call_inputs)?
+        } else {
+            convert_string_inputs(foreign_call_inputs)?
+        };
+        println!("{output_string}");
         Ok(())
     }
 }
 
-/// Fetch the abi type from the foreign call input
-/// The remaining input values should hold the values to be printed
-fn fetch_abi_type(
-    foreign_call_inputs: &[Vec<Value>],
-) -> Result<(AbiType, &[Vec<Value>]), ForeignCallError> {
+fn convert_string_inputs(foreign_call_inputs: &[Vec<Value>]) -> Result<String, ForeignCallError> {
+    // Fetch the abi type from the foreign call input
+    // The remaining input values should hold what is to be printed
     let (abi_type_as_values, input_values) =
         foreign_call_inputs.split_last().ok_or(ForeignCallError::MissingForeignCallInputs)?;
+    let abi_type = fetch_abi_type(abi_type_as_values)?;
+
+    // We must use a flat map here as each value in a struct will be in a separate input value
+    let mut input_values_as_fields =
+        input_values.iter().flat_map(|values| vecmap(values, |value| value.to_field()));
+
+    let input_value_display =
+        InputValueDisplay::try_from_fields(&mut input_values_as_fields, abi_type)?;
+
+    Ok(input_value_display.to_string())
+}
+
+fn convert_fmt_string_inputs(
+    foreign_call_inputs: &[Vec<Value>],
+) -> Result<String, ForeignCallError> {
+    let (message_as_values, input_and_abi_values) =
+        foreign_call_inputs.split_first().ok_or(ForeignCallError::MissingForeignCallInputs)?;
+
+    let message_as_fields = vecmap(message_as_values, |value| value.to_field());
+    let message_as_string = decode_string_value(&message_as_fields);
+
+    let (num_values, input_and_abi_values) =
+        input_and_abi_values.split_first().ok_or(ForeignCallError::MissingForeignCallInputs)?;
+
+    let mut output_strings = Vec::new();
+    let num_values = num_values[0].to_field().to_u128() as usize;
+
+    let mut abi_types = Vec::new();
+    for abi_values in input_and_abi_values.iter().skip(input_and_abi_values.len() - num_values) {
+        let abi_type = fetch_abi_type(abi_values)?;
+        abi_types.push(abi_type);
+    }
+
+    for i in 0..num_values {
+        let abi_type = &abi_types[i];
+        let type_size = abi_type.field_count() as usize;
+
+        let mut input_values_as_fields = input_and_abi_values[i..(i + type_size)]
+            .iter()
+            .flat_map(|values| vecmap(values, |value| value.to_field()));
+
+        let input_value_display =
+            InputValueDisplay::try_from_fields(&mut input_values_as_fields, abi_type.clone())?;
+
+        output_strings.push(input_value_display.to_string());
+    }
+
+    let mut output_strings_iter = output_strings.into_iter();
+    let re = Regex::new(r"\{([a-zA-Z0-9_]+)\}")
+        .expect("ICE: an invalid regex pattern was used for checking format strings");
+
+    let formatted_str = re.replace_all(&message_as_string, |_: &Captures| {
+        output_strings_iter
+            .next()
+            .expect("ICE: there are more regex matches than fields supplied to the format string")
+    });
+
+    Ok(formatted_str.into_owned())
+}
+
+fn fetch_abi_type(abi_type_as_values: &[Value]) -> Result<AbiType, ForeignCallError> {
     let abi_type_as_fields = vecmap(abi_type_as_values, |value| value.to_field());
     let abi_type_as_string = decode_string_value(&abi_type_as_fields);
     let abi_type: AbiType = serde_json::from_str(&abi_type_as_string)
         .map_err(|err| ForeignCallError::InputParserError(err.into()))?;
 
-    Ok((abi_type, input_values))
+    Ok(abi_type)
 }

crates/nargo_cli/tests/test_data/debug_logs/src/main.nr

@@ -1,14 +1,56 @@
 use dep::std;
 
 fn main(x : Field, y : pub Field) {
+    let string = "i: {i}, j: {j}";
+    std::println(string);
+
+    // A `fmtstr` lets you easily perform string interpolation.
+    let fmt_str: fmtstr<14, (Field, Field)> = f"i: {x}, j: {y}";
+    let fmt_str = string_identity(fmt_str);
+    std::println(fmt_str);
+
+    let fmt_str_no_type = f"i: {x}, j: {y}";
+    std::println(fmt_str_no_type);
+
+    let fmt_str_generic = string_with_generics(fmt_str_no_type);
+    std::println(fmt_str_generic);
+
+    let s = myStruct { y: x, x: y };
+    std::println(s);
+
+    std::println(f"randomstring{x}{x}");
+
+    let fmt_str = string_with_partial_generics(f"i: {x}, s: {s}");
+    std::println(fmt_str);
 
-    std::println("*** println ***");
     std::println(x);
     std::println([x, y]);
 
-    let s = myStruct { y: x, x: y };
     let foo = fooStruct { my_struct: s, foo: 15 };
-    std::println(foo);
+    std::println(f"s: {s}, foo: {foo}");
+
+    std::println(f"x: 0, y: 1");
+
+    let s_2 = myStruct { x: 20, y: 30 };
+    std::println(f"s1: {s}, s2: {s_2}");
+
+    let bar = fooStruct { my_struct: s_2, foo: 20 };
+    std::println(f"foo1: {foo}, foo2: {bar}");
+
+    let struct_string = if x != 5 { f"{foo}" } else { f"{bar}" };
+    std::println(struct_string);
+}
+
+fn string_identity(string: fmtstr<14, (Field, Field)>) -> fmtstr<14, (Field, Field)> {
+    string
+}
+
+fn string_with_generics<N, T>(string: fmtstr<N, T>) -> fmtstr<N, T> {
+    string
+}
+
+fn string_with_partial_generics<N, T>(string: fmtstr<N, (Field, T)>) -> fmtstr<N, (Field, T)> {
+    string
 }
 
 struct myStruct {

crates/noirc_evaluator/src/ssa_refactor/ssa_gen/context.rs

@@ -177,6 +177,15 @@ impl<'a> FunctionContext<'a> {
             ast::Type::MutableReference(element) => {
                 Self::map_type_helper(element, &mut |_| f(Type::Reference))
             }
+            ast::Type::FmtString(len, fields) => {
+                // A format string is represented by multiple values
+                // The message string, the number of fields to be formatted, and
+                // then the encapsulated fields themselves
+                let final_fmt_str_fields =
+                    vec![ast::Type::String(*len), ast::Type::Field, *fields.clone()];
+                let fmt_str_tuple = ast::Type::Tuple(final_fmt_str_fields);
+                Self::map_type_helper(&fmt_str_tuple, f)
+            }
             other => Tree::Leaf(f(Self::convert_non_tuple_type(other))),
         }
     }
@@ -204,6 +213,9 @@ impl<'a> FunctionContext<'a> {
             ast::Type::Integer(Signedness::Unsigned, bits) => Type::unsigned(*bits),
             ast::Type::Bool => Type::unsigned(1),
             ast::Type::String(len) => Type::Array(Rc::new(vec![Type::char()]), *len as usize),
+            ast::Type::FmtString(_, _) => {
+                panic!("convert_non_tuple_type called on a fmt string: {typ}")
+            }
             ast::Type::Unit => panic!("convert_non_tuple_type called on a unit type"),
             ast::Type::Tuple(_) => panic!("convert_non_tuple_type called on a tuple: {typ}"),
             ast::Type::Function(_, _) => Type::Function,

crates/noirc_evaluator/src/ssa_refactor/ssa_gen/mod.rs

@@ -125,6 +125,18 @@ impl<'a> FunctionContext<'a> {
                 let typ = Self::convert_non_tuple_type(&ast::Type::String(elements.len() as u64));
                 self.codegen_array(elements, typ)
             }
+            ast::Literal::FmtStr(string, number_of_fields, fields) => {
+                // A caller needs multiple pieces of information to make use of a format string
+                // The message string, the number of fields to be formatted, and the fields themselves
+                let string = Expression::Literal(ast::Literal::Str(string.clone()));
+                let number_of_fields = Expression::Literal(ast::Literal::Integer(
+                    (*number_of_fields as u128).into(),
+                    ast::Type::Field,
+                ));
+                let fields = *fields.clone();
+                let fmt_str_tuple = &[string, number_of_fields, fields];
+                self.codegen_tuple(fmt_str_tuple)
+            }
         }
     }
 

crates/noirc_frontend/Cargo.toml

@@ -20,6 +20,7 @@ serde.workspace = true
 serde_json.workspace = true
 rustc-hash = "1.1.0"
 small-ord-set = "0.1.3"
+regex = "1.9.1"
 
 [dev-dependencies]
 strum = "0.24"

crates/noirc_frontend/src/ast/expression.rs

@@ -72,6 +72,10 @@ impl ExpressionKind {
         ExpressionKind::Literal(Literal::Str(contents))
     }
 
+    pub fn format_string(contents: String) -> ExpressionKind {
+        ExpressionKind::Literal(Literal::FmtStr(contents))
+    }
+
     pub fn constructor((type_name, fields): (Path, Vec<(Ident, Expression)>)) -> ExpressionKind {
         ExpressionKind::Constructor(Box::new(ConstructorExpression { type_name, fields }))
     }
@@ -298,6 +302,7 @@ pub enum Literal {
     Bool(bool),
     Integer(FieldElement),
     Str(String),
+    FmtStr(String),
     Unit,
 }
 
@@ -473,6 +478,7 @@ impl Display for Literal {
             Literal::Bool(boolean) => write!(f, "{}", if *boolean { "true" } else { "false" }),
             Literal::Integer(integer) => write!(f, "{}", integer.to_u128()),
             Literal::Str(string) => write!(f, "\"{string}\""),
+            Literal::FmtStr(string) => write!(f, "f\"{string}\""),
             Literal::Unit => write!(f, "()"),
         }
     }

crates/noirc_frontend/src/ast/mod.rs

@@ -36,6 +36,7 @@ pub enum UnresolvedType {
     Bool(CompTime),
     Expression(UnresolvedTypeExpression),
     String(Option<UnresolvedTypeExpression>),
+    FormatString(UnresolvedTypeExpression, Box<UnresolvedType>),
     Unit,
 
     /// A Named UnresolvedType can be a struct type or a type variable
@@ -102,9 +103,10 @@ impl std::fmt::Display for UnresolvedType {
             Expression(expression) => expression.fmt(f),
             Bool(is_const) => write!(f, "{is_const}bool"),
             String(len) => match len {
-                None => write!(f, "str[]"),
-                Some(len) => write!(f, "str[{len}]"),
+                None => write!(f, "str<_>"),
+                Some(len) => write!(f, "str<{len}>"),
             },
+            FormatString(len, elements) => write!(f, "fmt<{len}, {elements}"),
             Function(args, ret) => {
                 let args = vecmap(args, ToString::to_string);
                 write!(f, "fn({}) -> {ret}", args.join(", "))

crates/noirc_frontend/src/hir/resolution/errors.rs

@@ -74,6 +74,8 @@ pub enum ResolverError {
     MutableReferenceToArrayElement { span: Span },
     #[error("Function is not defined in a contract yet sets is_internal")]
     ContractFunctionInternalInNormalFunction { span: Span },
+    #[error("Numeric constants should be printed without formatting braces")]
+    NumericConstantInFormatString { name: String, span: Span },
 }
 
 impl ResolverError {
@@ -283,6 +285,11 @@ impl From<ResolverError> for Diagnostic {
                 "Non-contract functions cannot be 'internal'".into(),
                 span,
             ),
+            ResolverError::NumericConstantInFormatString { name, span } => Diagnostic::simple_error(
+                format!("cannot find `{name}` in this scope "),
+                "Numeric constants should be printed without formatting braces".to_string(),
+                span,
+            ),
         }
     }
 }