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Verbose Syntax (F#)

There are two forms of syntax available for many constructs in the F# language: verbose syntax and lightweight syntax. The verbose syntax is not as commonly used, but has the advantage of being less sensitive to indentation. The lightweight syntax is shorter and uses indentation to signal the beginning and end of constructs, rather than additional keywords like begin, end, in, and so on. The default syntax is the lightweight syntax. This topic describes the syntax for F# constructs when lightweight syntax is not enabled. Verbose syntax is always enabled, so even if you enable lightweight syntax, you can still use verbose syntax for some constructs. You can disable lightweight syntax by using the #light "off" directive.

The following table shows the lightweight and verbose syntax for F# language constructs in contexts where there is a difference between the two forms. In this table, angle brackets (<>) enclose user-supplied syntax elements. Refer to the documentation for each language construct for more detailed information about the syntax used within these constructs.

Language construct

Lightweight syntax

Verbose syntax

compound expressions

<expression1>
<expression2>
<expression1>; <expression2>

nested let bindings

let f x =
    let a = 1
    let b = 2
    x + a + b
let f x =
    let a = 1 in
    let b = 2 in
    x + a + b

code block

    <expression1>
    <expression2>
    ...
    begin
        <expression1>;
        <expression2>;
    end

for...do

for counter = start to finish do
    ...
for counter = start .. finish do
    ...
    done

while...do

while <condition> do
    ...
while <condition> do
    ...
    done

for...in

for var in start .. finish do
    ...
for var in start .. finish do
    ...
    done

do

do ...
do ... in

record

type <record-name> =
    {
        <field-declarations>
    }
    <value-or-member-definitions>
type <record-name> =
    {
        <field-declarations>
    }
    with
        <value-or-member-definitions>
    end

class

type <class-name>(<params>) =
    ...
type <class-name>(<params>) =
    class
        ...
    end

structure

[<StructAttribute>]
type <structure-name> =
    ...
type <structure-name> =
    struct
        ...
    end

discriminated union

type <union-name> =
    | ...
    | ...
    ...
    <value-or-member definitions>
type <union-name> =
    | ...
    | ...
    ...
    with
         <value-or-member-definitions>

end

interface

type <interface-name> =
    ...
type <interface-name> =
    interface
        ...
    end

object expression

{ new <type-name>
    with
        <value-or-member-definitions>
    <interface-implementations>
}
{ new <type-name>
    with
        <value-or-member-definitions>
    end
    <interface-implementations>
}

interface implementation

interface <interface-name>
    with
        <value-or-member-definitions>
interface <interface-name>
    with
        <value-or-member-definitions>
    end

type extension

type <type-name>
    with
        <value-or-member-definitions>
type <type-name>
    with
        <value-or-member-definitions>
    end

module

module <module-name> =
    ...
module <module-name> =
    begin
        ...
    end

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