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"