BinaryFormat.Choice

Returns a binary format that chooses the next binary format based on a value that has already been read.

Syntax

BinaryFormat.Choice(
    binaryFormat as function,
    chooseFunction as function,
    optional type as type,
    optional combineFunction as function
) as function

Remarks

Returns a binary format that chooses the next binary format based on a value that has already been read. The binary format value produced by this function works in stages:

• The binary format specified by the binaryFormat parameter is used to read a value.
• The value is passed to the choice function specified by the chooseFunction parameter.
• The choice function inspects the value and returns a second binary format.
• The second binary format is used to read a second value.
• If the combine function is specified, then the first and second values are passed to the combine function, and the resulting value is returned.
• If the combine function is not specified, the second value is returned.
• The second value is returned.

The optional type parameter indicates the type of binary format that will be returned by the choice function. Either type any, type list, or type binary may be specified. If the type parameter is not specified, then type any is used. If type list or type binary is used, then the system may be able to return a streaming binary or list value instead of a buffered one, which may reduce the amount of memory necessary to read the format.

Examples

Example #1

Read a list of bytes where the number of elements is determined by the first byte.

let
    binaryData = #binary({2, 3, 4, 5}),
    listFormat = BinaryFormat.Choice(
        BinaryFormat.Byte,
        (length) => BinaryFormat.List(BinaryFormat.Byte, length)
    )
in
    listFormat(binaryData)

Result:

{3, 4}

Example #2

Read a list of bytes where the number of elements is determined by the first byte, and preserve the first byte read.

let
    binaryData = #binary({2, 3, 4, 5}),
    listFormat = BinaryFormat.Choice(
        BinaryFormat.Byte,
        (length) => BinaryFormat.Record([
            length = length,
            list = BinaryFormat.List(BinaryFormat.Byte, length)
        ])
    )
in
    listFormat(binaryData)

Result:

[length = 2, list = {3, 4}]

Example #3

Read a list of bytes where the number of elements is determined by the first byte using a streaming list.

let
    binaryData = #binary({2, 3, 4, 5}),
    listFormat = BinaryFormat.Choice(
        BinaryFormat.Byte,
        (length) => BinaryFormat.List(BinaryFormat.Byte, length),
        type list
    )
in
    listFormat(binaryData)

Result:

{3, 4}

Category

Binary Formats.Controlling what comes next