Type.ForFunction

Type

Creates a function type from a signature record and minimum parameter count.

Syntax

Type.ForFunction(signature as record, min as number) as type

Parameters

Name	Type	Required	Description
`signature`	`record`	Yes	A record with ReturnType (a type) and Parameters (a list of records with Name, Type, and optional Description).
`min`	`number`	Yes	The minimum number of required parameters.

Return Value

type — A function type matching the specified signature.

Remarks

Type.ForFunction constructs a function type from a descriptive record. The signature record has two fields:

ReturnType — the return type of the function.
Parameters — a list of records, each with Name (text), Type (type), and optionally Description (text).

The min parameter specifies how many of the parameters are required. Parameters beyond min are treated as optional.

This function is typically used with Function.From to create dynamically typed custom functions, or with Value.ReplaceType to add parameter metadata to an existing function so it renders properly in the Power Query UI.

Typical pattern

let
    Signature = [
        ReturnType = type text,
        Parameters = {
            [Name = "input", Type = type text, Description = "The input text"],
            [Name = "prefix", Type = type text, Description = "Optional prefix"]
        }
    ],
    FnType = Type.ForFunction(Signature, 1)  // 1 required, 1 optional
in
    FnType

Examples

Example 1: Create a function type and verify it

let
    Signature = [
        ReturnType = type number,
        Parameters = {
            [Name = "a", Type = type number],
            [Name = "b", Type = type number]
        }
    ],
    FnType = Type.ForFunction(Signature, 2),
    IsFunction = Type.Is(FnType, type function)
in
    #table({"Description", "IsFunction"}, {{"type function (a as number, b as number) as number", IsFunction}})

Applied Steps

The final output — presents a table with a description of the function type and the IsFunction logical confirmation.

	Description	IsFunction
1	type function (a as number, b as number) as number	TRUE

Compatibility

✓ Power BI Desktop✓ Power BI Service✓ Excel Desktop✓ Excel Online✓ Dataflows✓ Fabric Notebooks

Contributors