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Flexible Types (F#)

A flexible type annotation indicates that a parameter, variable, or value has a type that is compatible with a specifed type, where compatibility is determined by position in an object-oriented hierarchy of classes or interfaces. Flexible types are useful specifically when the automatic conversion to types higher in the type hierarchy does not occur but you still want to enable your functionality to work with any type in the hierarchy or any type that implements an interface.

#type

In the previous syntax, type represents a base type or an interface.

A flexible type is equivalent to a generic type that has a constraint that limits the allowed types to types that are compatible with the base or interface type. That is, the following two lines of code are equivalent.

#SomeType

'a when 'a :> SomeType

Flexible types are useful in several types of situations. For example, when you have a higher order function (a function that takes a function as an argument), it is often useful to have the function return a flexible type. In the following example, the use of a flexible type with a sequence argument in iterate2 enables the higher order function to work with functions that generate sequences, arrays, lists, and any other enumerable type.

Consider the following two functions, one of which returns a sequence, the other of which returns a flexible type.

let iterate1 (f : unit -> seq<int>) =
    for e in f() do printfn "%d" e
let iterate2 (f : unit -> #seq<int>) =
    for e in f() do printfn "%d" e

// Passing a function that takes a list requires a cast.
iterate1 (fun () -> [1] :> seq<int>)

// Passing a function that takes a list to the version that specifies a 
// flexible type as the return value is OK as is.
iterate2 (fun () -> [1])

As another example, consider the Seq.concat library function:

val concat: sequences:seq<#seq<'T>> -> seq<'T>

You can pass any of the following enumerable sequences to this function:

  • A list of lists

  • A list of arrays

  • An array of lists

  • An array of sequences

  • Any other combination of enumerable sequences

The following code uses Seq.concat to demonstrate the scenarios that you can support by using flexible types.

let list1 = [1;2;3]
let list2 = [4;5;6]
let list3 = [7;8;9]

let concat1 = Seq.concat [ list1; list2; list3]
printfn "%A" concat1

let array1 = [|1;2;3|]
let array2 = [|4;5;6|]
let array3 = [|7;8;9|]

let concat2 = Seq.concat [ array1; array2; array3 ]
printfn "%A" concat2

let concat3 = Seq.concat [| list1; list2; list3 |]
printfn "%A" concat3

let concat4 = Seq.concat [| array1; array2; array3 |]
printfn "%A" concat4

let seq1 = { 1 .. 3 }
let seq2 = { 4 .. 6 }
let seq3 = { 7 .. 9 }

let concat5 = Seq.concat [| seq1; seq2; seq3 |]

printfn "%A" concat5

The output is as follows.

seq [1; 2; 3; 4; ...]
seq [1; 2; 3; 4; ...]
seq [1; 2; 3; 4; ...]
seq [1; 2; 3; 4; ...]
seq [1; 2; 3; 4; ...]

In F#, as in other object-oriented languages, there are contexts in which derived types or types that implement interfaces are automatically converted to a base type or interface type. These automatic conversions occur in direct arguments, but not when the type is in a subordinate position, as part of a more complex type such as a return type of a function type, or as a type argument. Thus, the flexible type notation is primarily useful when the type you are applying it to is part of a more complex type.

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