# Lambda improvements

## Summary

Proposed changes:

1. Allow lambdas with attributes
2. Allow lambdas with explicit return type
3. Infer a natural delegate type for lambdas and method groups

## Motivation

Support for attributes on lambdas would provide parity with methods and local functions.

Support for explicit return types would provide symmetry with lambda parameters where explicit types can be specified. Allowing explicit return types would also provide control over compiler performance in nested lambdas where overload resolution must bind the lambda body currently to determine the signature.

A natural type for lambda expressions and method groups will allow more scenarios where lambdas and method groups may be used without an explicit delegate type, including as initializers in var declarations.

Requiring explicit delegate types for lambdas and method groups has been a friction point for customers, and has become an impediment to progress in ASP.NET with recent work on MapAction.

ASP.NET MapAction without proposed changes (MapAction() takes a System.Delegate argument):

[HttpGet("/")] Todo GetTodo() => new(Id: 0, Name: "Name");
app.MapAction((Func<Todo>)GetTodo);

[HttpPost("/")] Todo PostTodo([FromBody] Todo todo) => todo;
app.MapAction((Func<Todo, Todo>)PostTodo);


ASP.NET MapAction with natural types for method groups:

[HttpGet("/")] Todo GetTodo() => new(Id: 0, Name: "Name");
app.MapAction(GetTodo);

[HttpPost("/")] Todo PostTodo([FromBody] Todo todo) => todo;
app.MapAction(PostTodo);


ASP.NET MapAction with attributes and natural types for lambda expressions:

app.MapAction([HttpGet("/")] () => new Todo(Id: 0, Name: "Name"));
app.MapAction([HttpPost("/")] ([FromBody] Todo todo) => todo);


## Attributes

Attributes may be added to lambda expressions and lambda parameters. To avoid ambiguity between method attributes and parameter attributes, a lambda expression with attributes must use a parenthesized parameter list. Parameter types are not required.

f = [A] () => { };        // [A] lambda
f = [return:A] x => x;    // syntax error at '=>'
f = [return:A] (x) => x;  // [A] lambda
f = [A] static x => x;    // syntax error at '=>'

f = ([A] x) => x;         // [A] x
f = ([A] ref int x) => x; // [A] x


Multiple attributes may be specified, either comma-separated within the same attribute list or as separate attribute lists.

var f = [A1, A2][A3] () => { };    // ok
var g = ([A1][A2, A3] int x) => x; // ok


Attributes are not supported for anonymous methods declared with delegate { } syntax.

f = [A] delegate { return 1; };         // syntax error at 'delegate'
f = delegate ([A] int x) { return x; }; // syntax error at '['


The parser will look ahead to differentiate a collection initializer with an element assignment from a collection initializer with a lambda expression.

var y = new C { [A] = x };    // ok: y[A] = x
var z = new C { [A] x => x }; // ok: z[0] = [A] x => x


The parser will treat ?[ as the start of a conditional element access.

x = b ? [A];               // ok
y = b ? [A] () => { } : z; // syntax error at '('


Attributes on the lambda expression or lambda parameters will be emitted to metadata on the method that maps to the lambda.

In general, customers should not depend on how lambda expressions and local functions map from source to metadata. How lambdas and local functions are emitted can, and has, changed between compiler versions.

The changes proposed here are targeted at the Delegate driven scenario. It should be valid to inspect the MethodInfo associated with a Delegate instance to determine the signature of the lambda expression or local function including any explicit attributes and additional metadata emitted by the compiler such as default parameters. This allows teams such as ASP.NET to make available the same behaviors for lambdas and local functions as ordinary methods.

## Explicit return type

An explicit return type may be specified before the parenthesized parameter list.

f = T () => default;                    // ok
f = short x => 1;                       // syntax error at '=>'
f = ref int (ref int x) => ref x;       // ok
f = static void (_) => { };             // ok
f = async async (async async) => async; // ok?


The parser will look ahead to differentiate a method call T() from a lambda expression T () => e.

Explicit return types are not supported for anonymous methods declared with delegate { } syntax.

f = delegate int { return 1; };         // syntax error
f = delegate int (int x) { return x; }; // syntax error


Method type inference should make an exact inference from an explicit lambda return type.

static void F<T>(Func<T, T> f) { ... }
F(int (i) => i); // Func<int, int>


Variance conversions are not allowed from lambda return type to delegate return type (matching similar behavior for parameter types).

Func<object> f1 = string () => null; // error
Func<object?> f2 = object () => x;   // warning


The parser allows lambda expressions with ref return types within expressions without additional parentheses.

d = ref int () => x; // d = (ref int () => x)
F(ref int () => x);  // F((ref int () => x))


var cannot be used as an explicit return type for lambda expressions.

class var { }

d = var (var v) => v;              // error: contextual keyword 'var' cannot be used as explicit lambda return type
d = @var (var v) => v;             // ok
d = ref var (ref var v) => ref v;  // error: contextual keyword 'var' cannot be used as explicit lambda return type
d = ref @var (ref var v) => ref v; // ok


## Natural (function) type

An anonymous function expression (§11.16) (a lambda expression or an anonymous method) has a natural type if the parameters types are explicit and the return type is either explicit or can be inferred (see §11.6.3.13).

A method group has a natural type if all candidate methods in the method group have a common signature. (If the method group may include extension methods, the candidates include the containing type and all extension method scopes.)

The natural type of an anonymous function expression or method group is a function_type. A function_type represents a method signature: the parameter types and ref kinds, and return type and ref kind. Anonymous function expressions or method groups with the same signature have the same function_type.

Function_types are used in a few specific contexts only:

• implicit and explicit conversions
• method type inference (§11.6.3) and best common type (§11.6.3.15)
• var initializers

A function_type exists at compile time only: function_types do not appear in source or metadata.

### Conversions

From a function_type F there are implicit function_type conversions:

• To a function_type G if the parameters and return types of F are variance-convertible to the parameters and return type of G
• To System.MulticastDelegate or base classes or interfaces of System.MulticastDelegate
• To System.Linq.Expressions.Expression or System.Linq.Expressions.LambdaExpression

Anonymous function expressions and method groups already have conversions from expression to delegate types and expression tree types (see anonymous function conversions §10.7 and method group conversions §10.8). Those conversions are sufficient for converting to strongly-typed delegate types and expression tree types. The function_type conversions above add conversions from type to the base types only: System.MulticastDelegate, System.Linq.Expressions.Expression, etc.

There are no conversions to a function_type from a type other than a function_type. There are no explicit conversions for function_types since function_types cannot be referenced in source.

A conversion to System.MulticastDelegate or base type or interface realizes the anonymous function or method group as an instance of an appropriate delegate type. A conversion to System.Linq.Expressions.Expression<TDelegate> or base type realizes the lambda expression as an expression tree with an appropriate delegate type.

Delegate d = delegate (object obj) { }; // Action<object>
Expression e = () => "";                // Expression<Func<string>>
object o = "".Clone;                    // Func<object>


Function_type conversions are not implicit or explicit standard conversions §10.4 and are not considered when determining whether a user-defined conversion operator is applicable to an anonymous function or method group. From evaluation of user defined conversions §10.5.3:

For a conversion operator to be applicable, it must be possible to perform a standard conversion (§10.4) from the source type to the operand type of the operator, and it must be possible to perform a standard conversion from the result type of the operator to the target type.

class C
{
public static implicit operator C(Delegate d) { ... }
}

C c;
c = () => 1;      // error: cannot convert lambda expression to type 'C'
c = (C)(() => 2); // error: cannot convert lambda expression to type 'C'


A warning is reported for an implicit conversion of a method group to object, since the conversion is valid but perhaps unintentional.

Random r = new Random();
object obj;
obj = r.NextDouble;         // warning: Converting method group to 'object'. Did you intend to invoke the method?
obj = (object)r.NextDouble; // ok


### Type inference

The existing rules for type inference are mostly unchanged (see §11.6.3). There are however a couple of changes below to specific phases of type inference.

#### First phase

The first phase (§11.6.3.2) allows an anonymous function to bind to Ti even if Ti is not a delegate or expression tree type (perhaps a type parameter constrained to System.Delegate for instance).

For each of the method arguments Ei:

• If Ei is an anonymous function and Ti is a delegate type or expression tree type, an explicit parameter type inference is made from Ei to Ti and an explicit return type inference is made from Ei to Ti.
• Otherwise, if Ei has a type U and xi is a value parameter then a lower-bound inference is made from U to Ti.
• Otherwise, if Ei has a type U and xi is a ref or out parameter then an exact inference is made from U to Ti.
• Otherwise, no inference is made for this argument.

#### Explicit return type inference

An explicit return type inference is made from an expression E to a type T in the following way:

• If E is an anonymous function with explicit return type Ur and T is a delegate type or expression tree type with return type Vr then an exact inference (§11.6.3.9) is made from Ur to Vr.

#### Fixing

Fixing (§11.6.3.12) ensures other conversions are preferred over function_type conversions. (Lambda expressions and method group expressions only contribute to lower bounds so handling of function_types is needed for lower bounds only.)

An unfixed type variable Xi with a set of bounds is fixed as follows:

• The set of candidate types Uj starts out as the set of all types in the set of bounds for Xi where function types are ignored in lower bounds if there any types that are not function types.
• We then examine each bound for Xi in turn: For each exact bound U of Xi all types Uj which are not identical to U are removed from the candidate set. For each lower bound U of Xi all types Uj to which there is not an implicit conversion from U are removed from the candidate set. For each upper bound U of Xi all types Uj from which there is not an implicit conversion to U are removed from the candidate set.
• If among the remaining candidate types Uj there is a unique type V from which there is an implicit conversion to all the other candidate types, then Xi is fixed to V.
• Otherwise, type inference fails.

### Best common type

Best common type (§11.6.3.15) is defined in terms of type inference so the type inference changes above apply to best common type as well.

var fs = new[] { (string s) => s.Length; (string s) => int.Parse(s) } // Func<string, int>[]


### var

Anonymous functions and method groups with function types can be used as initializers in var declarations.

var f1 = () => default;           // error: cannot infer type
var f2 = x => x;                  // error: cannot infer type
var f3 = () => 1;                 // System.Func<int>
var f4 = string () => null;       // System.Func<string>
var f5 = delegate (object o) { }; // System.Action<object>

static void F1() { }
static void F1<T>(this T t) { }
static void F2(this string s) { }

var f6 = F1;    // error: multiple methods
var f7 = "".F1; // System.Action
var f8 = F2;    // System.Action<string>


Function types are not used in assignments to discards.

d = () => 0; // ok
_ = () => 1; // error


### Delegate types

The delegate type for the anonymous function or method group with parameter types P1, ..., Pn and return type R is:

• if any parameter or return value is not by value, or there are more than 16 parameters, or any of the parameter types or return are not valid type arguments (say, (int* p) => { }), then the delegate is a synthesized internal anonymous delegate type with signature that matches the anonymous function or method group, and with parameter names arg1, ..., argn or arg if a single parameter;
• if R is void, then the delegate type is System.Action<P1, ..., Pn>;
• otherwise the delegate type is System.Func<P1, ..., Pn, R>.

The compiler may allow more signatures to bind to System.Action<> and System.Func<> types in the future (if ref struct types are allowed type arguments for instance).

modopt() or modreq() in the method group signature are ignored in the corresponding delegate type.

If two anonymous functions or method groups in the same compilation require synthesized delegate types with the same parameter types and modifiers and the same return type and modifiers, the compiler will use the same synthesized delegate type.

Better function member (§11.6.4.3) is updated to prefer members where none of the conversions and none of the type arguments involved inferred types from lambda expressions or method groups.

#### Better function member

... Given an argument list A with a set of argument expressions {E1, E2, ..., En} and two applicable function members Mp and Mq with parameter types {P1, P2, ..., Pn} and {Q1, Q2, ..., Qn}, Mp is defined to be a better function member than Mq if

1. for each argument, the implicit conversion from Ex to Px is not a function_type_conversion, and
• Mp is a non-generic method or Mp is a generic method with type parameters {X1, X2, ..., Xp} and for each type parameter Xi the type argument is inferred from an expression or from a type other than a function_type, and
• for at least one argument, the implicit conversion from Ex to Qx is a function_type_conversion, or Mq is a generic method with type parameters {Y1, Y2, ..., Yq} and for at least one type parameter Yi the type argument is inferred from a function_type, or
2. for each argument, the implicit conversion from Ex to Qx is not better than the implicit conversion from Ex to Px, and for at least one argument, the conversion from Ex to Px is better than the conversion from Ex to Qx.

Better conversion from expression (§11.6.4.4) is updated to prefer conversions that did not involve inferred types from lambda expressions or method groups.

#### Better conversion from expression

Given an implicit conversion C1 that converts from an expression E to a type T1, and an implicit conversion C2 that converts from an expression E to a type T2, C1 is a better conversion than C2 if:

1. C1 is not a function_type_conversion and C2 is a function_type_conversion, or
2. E is a non-constant interpolated_string_expression, C1 is an implicit_string_handler_conversion, T1 is an applicable_interpolated_string_handler_type, and C2 is not an implicit_string_handler_conversion, or
3. E does not exactly match T2 and at least one of the following holds:
• E exactly matches T1 (§11.6.4.4)
• T1 is a better conversion target than T2 (§11.6.4.6)

## Syntax

lambda_expression
: modifier* identifier '=>' (block | expression)
| attribute_list* modifier* type? lambda_parameters '=>' (block | expression)
;

lambda_parameters
: lambda_parameter
| '(' (lambda_parameter (',' lambda_parameter)*)? ')'
;

lambda_parameter
: identifier
| attribute_list* modifier* type? identifier equals_value_clause?
;


## Open issues

Should default values be supported for lambda expression parameters for completeness?

Should System.Diagnostics.ConditionalAttribute be disallowed on lambda expressions since there are few scenarios where a lambda expression could be used conditionally?

([Conditional("DEBUG")] static (x, y) => Assert(x == y))(a, b); // ok?


Should the function_type be available from the compiler API, in addition to the resulting delegate type?

Currently, the inferred delegate type uses System.Action<> or System.Func<> when parameter and return types are valid type arguments and there are no more than 16 parameters, and if the expected Action<> or Func<> type is missing, an error is reported. Instead, should the compiler use System.Action<> or System.Func<> regardless of arity? And if the expected type is missing, synthesize a delegate type otherwise?