Den Delimarsky

F# function values, methods, properties, and aggregate types such as classes, records, and discriminated unions can be generic. Generic constructs contain at least one type parameter, which is usually supplied by the user of the generic construct. Generic functions and types enable you to write code that works with a variety of types without repeating the code for each type. Making your code generic can be simple in F#, because often your code is implicitly inferred to be generic by the compiler's type inference and automatic generalization mechanisms.


// Explicitly generic function.
let function-name<type-parameters> parameter-list =

// Explicitly generic method.
[ static ] member object-identifer.method-name<type-parameters> parameter-list [ return-type ] =

// Explicitly generic class, record, interface, structure,
// or discriminated union.
type type-name<type-parameters> type-definition


The declaration of an explicitly generic function or type is much like that of a non-generic function or type, except for the specification (and use) of the type parameters, in angle brackets after the function or type name.

Declarations are often implicitly generic. If you do not fully specify the type of every parameter that is used to compose a function or type, the compiler attempts to infer the type of each parameter, value, and variable from the code you write. For more information, see Type Inference. If the code for your type or function does not otherwise constrain the types of parameters, the function or type is implicitly generic. This process is named automatic generalization. There are some limits on automatic generalization. For example, if the F# compiler is unable to infer the types for a generic construct, the compiler reports an error that refers to a restriction called the value restriction. In that case, you may have to add some type annotations. For more information about automatic generalization and the value restriction, and how to change your code to address the problem, see Automatic Generalization.

In the previous syntax, type-parameters is a comma-separated list of parameters that represent unknown types, each of which starts with a single quotation mark, optionally with a constraint clause that further limits what types may be used for that type parameter. For the syntax for constraint clauses of various kinds and other information about constraints, see Constraints.

The type-definition in the syntax is the same as the type definition for a non-generic type. It includes the constructor parameters for a class type, an optional as clause, the equal symbol, the record fields, the inherit clause, the choices for a discriminated union, let and do bindings, member definitions, and anything else permitted in a non-generic type definition.

The other syntax elements are the same as those for non-generic functions and types. For example, object-identifier is an identifier that represents the containing object itself.

Properties, fields, and constructors cannot be more generic than the enclosing type. Also, values in a module cannot be generic.

Implicitly Generic Constructs

When the F# compiler infers the types in your code, it automatically treats any function that can be generic as generic. If you specify a type explicitly, such as a parameter type, you prevent automatic generalization.

In the following code example, makeList is generic, even though neither it nor its parameters are explicitly declared as generic.

let makeList a b =
    [a; b]

The signature of the function is inferred to be 'a -> 'a -> 'a list. Note that a and b in this example are inferred to have the same type. This is because they are included in a list together, and all elements of a list must be of the same type.

You can also make a function generic by using the single quotation mark syntax in a type annotation to indicate that a parameter type is a generic type parameter. In the following code, function1 is generic because its parameters are declared in this manner, as type parameters.

let function1 (x: 'a) (y: 'a) =
    printfn "%A %A" x y

Explicitly Generic Constructs

You can also make a function generic by explicitly declaring its type parameters in angle brackets (<type-parameter>). The following code illustrates this.

let function2<'T> x y =
    printfn "%A, %A" x y

Using Generic Constructs

When you use generic functions or methods, you might not have to specify the type arguments. The compiler uses type inference to infer the appropriate type arguments. If there is still an ambiguity, you can supply type arguments in angle brackets, separating multiple type arguments with commas.

The following code shows the use of the functions that are defined in the previous sections.

// In this case, the type argument is inferred to be int.
function1 10 20
// In this case, the type argument is float.
function1 10.0 20.0
// Type arguments can be specified, but should only be specified
// if the type parameters are declared explicitly. If specified,
// they have an effect on type inference, so in this example,
// a and b are inferred to have type int.
let function3 a b =
    // The compiler reports a warning:
    function1<int> a b
    // No warning.
    function2<int> a b

There are two ways to refer to a generic type by name. For example, list<int> and int list are two ways to refer to a generic type list that has a single type argument int. The latter form is conventionally used only with built-in F# types such as list and option. If there are multiple type arguments, you normally use the syntax Dictionary<int, string> but you can also use the syntax (int, string) Dictionary.

Wildcards as Type Arguments

To specify that a type argument should be inferred by the compiler, you can use the underscore, or wildcard symbol (_), instead of a named type argument. This is shown in the following code.

let printSequence (sequence1: Collections.seq<_>) =
   Seq.iter (fun elem -> printf "%s " (elem.ToString())) sequence1

Constraints in Generic Types and Functions

In a generic type or function definition, you can use only those constructs that are known to be available on the generic type parameter. This is required to enable the verification of function and method calls at compile time. If you declare your type parameters explicitly, you can apply an explicit constraint to a generic type parameter to notify the compiler that certain methods and functions are available. However, if you allow the F# compiler to infer your generic parameter types, it will determine the appropriate constraints for you. For more information, see Constraints.

Statically Resolved Type Parameters

There are two kinds of type parameters that can be used in F# programs. The first are generic type parameters of the kind described in the previous sections. This first kind of type parameter is equivalent to the generic type parameters that are used in languages such as Visual Basic and C#. Another kind of type parameter is specific to F# and is referred to as a statically resolved type parameter. For information about these constructs, see Statically Resolved Type Parameters.


// A generic function.
// In this example, the generic type parameter 'a makes function3 generic.
let function3 (x : 'a) (y : 'a) =
    printf "%A %A" x y

// A generic record, with the type parameter in angle brackets.
type GR<'a> =
        Field1: 'a;
        Field2: 'a;

// A generic class.
type C<'a>(a : 'a, b : 'a) =
    let z = a
    let y = b
    member this.GenericMethod(x : 'a) =
        printfn "%A %A %A" x y z

// A generic discriminated union.
type U<'a> =
    | Choice1 of 'a
    | Choice2 of 'a * 'a

type Test() =
    // A generic member
    member this.Function1<'a>(x, y) =
        printfn "%A, %A" x y

    // A generic abstract method.
    abstract abstractMethod<'a, 'b> : 'a * 'b -> unit
    override this.abstractMethod<'a, 'b>(x:'a, y:'b) =
         printfn "%A, %A" x y

See Also

Language Reference


Statically Resolved Type Parameters

Generics in the .NET Framework

Automatic Generalization