# Understanding Power Query M functions

In the Power Query M formula language, a *function* is a mapping from a set of input values to a single output value. A function is written by first naming the function parameters, and then providing an expression to compute the result of the function. The body of the function follows the goes-to (=>) symbol. Optionally, type information can be included on parameters and the function return value. A function is defined and invoked in the body of a **let** statement. Parameters and/or return value can be implicit or explicit. Implicit parameters and/or return value are of type **any**. Type **any** is similar to an object type in other languages. All types in M derive from type **any**.

A **function** is a value just like a number or a text value, and can be included in-line just like any other expression. The following example shows a function which is the value of an Add variable which is then invoked, or executed, from several other variables. When a function is invoked, a set of values are specified which are logically substituted for the required set of input values within the function body expression.

**Example – Explicit parameters and return value**

```
let
AddOne = (x as number) as number => x + 1,
//additional expression steps
CalcAddOne = AddOne(5)
in
CalcAddOne
```

**Example – Implicit parameters and return value**

```
let
Add = (x, y) => x + y,
AddResults =
[
OnePlusOne = Add(1, 1), // equals 2
OnePlusTwo = Add(1, 2) // equals 3
]
in
AddResults
```

**Find the first element of a list greater than 5, or null otherwise**

```
let
FirstGreaterThan5 = (list) =>
let
GreaterThan5 = List.Select(list, (n) => n> 5),
First = List.First(GreaterThan5)
in
First,
Results =
[
Found = FirstGreaterThan5({3,7,9}), // equals 7
NotFound = FirstGreaterThan5({1,3,4}) // equals null
]
in
Results
```

Functions can be used recursively. In order to recursively reference the function, prefix the identifier with @.

```
let
fact = (num) => if num = 0 then 1 else num * @fact (num-1)
in
fact(5) // equals 120
```

**Each keyword**

The **each** keyword is used to easily create simple functions. "each ..." is syntactic sugar for a function signature that takes the _ parameter "(_) => ..."

Each is useful when combined with the lookup operator, which is applied by default to _

For example, each [CustomerID] is the same as each _[CustomerID], which is the same as (_) => _[CustomerID]

**Example – Using each in table row filter**

```
Table.SelectRows(
Table.FromRecords({
[CustomerID = 1, Name = "Bob", Phone = "123-4567"],
[CustomerID = 2, Name = "Jim", Phone = "987-6543"] ,
[CustomerID = 3, Name = "Paul", Phone = "543-7890"] ,
[CustomerID = 4, Name = "Ringo", Phone = "232-1550"]
}),
each [CustomerID] = 2
)[Name]
// equals "Jim"
```

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