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Expressions, values, and let expression

  • Grein

A Power Query M formula language query is composed of formula expression steps that create a mashup query. A formula expression can be evaluated (computed), yielding a value. The let expression encapsulates a set of values to be computed, assigned names, and then used in a subsequent expression that follows the in statement. For example, a let expression could contain a Source variable that equals the value of Text.Proper and yields a text value in proper case.

Let expression

let
    Source = Text.Proper("hello world")
in
    Source

In the example above, Text.Proper("hello world") is evaluated to "Hello World".

The next sections describe value types in the language.

Primitive value

A primitive value is single-part value, such as a number, logical, text, or null. A null value can be used to indicate the absence of any data.

Type Example value
Binary 00 00 00 02 // number of points (2)
Date 5/23/2015
DateTime 5/23/2015 12:00:00 AM
DateTimeZone 5/23/2015 12:00:00 AM -08:00
Duration 15:35:00
Logical true and false
Null null
Number 0, 1, -1, 1.5, and 2.3e-5
Text "abc"
Time 12:34:12 PM

Function value

A Function is a value that, when invoked with arguments, produces a new value. Functions are written by listing the function's parameters in parentheses, followed by the goes-to symbol =>, followed by the expression defining the function. For example, to create a function called MyFunction that has two parameters and performs a calculation on parameter1 and parameter2:

let
    MyFunction = (parameter1, parameter2) => (parameter1 + parameter2) / 2
in
    MyFunction

Calling MyFunction returns the result:

let
    Source = MyFunction(2, 4)
in
    Source

This code produces the value of 3.

Structured data values

The M language supports the following structured data values:

Note

Structured data can contain any M value. To see a couple of examples, go to Additional structured data examples.

List

A List is a zero-based ordered sequence of values enclosed in curly brace characters { }. The curly brace characters { } are also used to retrieve an item from a List by index position. For more information, go to List values.

Note

Power Query M supports an infinite list size, but if a list is written as a literal, the list has a fixed length. For example, {1, 2, 3} has a fixed length of 3.

The following are some List examples.

Value Type
{123, true, "A"} List containing a number, a logical, and text.
{1, 2, 3} List of numbers
{
    {1, 2, 3},
    {4, 5, 6}
}		 List of List of numbers
{
    [CustomerID = 1, Name = "Bob", Phone = "123-4567"],
    [CustomerID = 2, Name = "Jim", Phone = "987-6543"]
}		 List of Records
{123, true, "A"}{0} Get the value of the first item in a List. This expression returns the value 123.
{
    {1, 2, 3},
    {4, 5, 6}
}{0}{1}		 Get the value of the second item from the first List element. This expression returns the value 2.

Record

A Record is a set of fields. A field is a name/value pair where the name is a text value that is unique within the field's record. The syntax for record values allows the names to be written without quotes, a form also referred to as identifiers. An identifier can take the following two forms:

  • identifier_name such as OrderID.

  • #"identifier name" such as #"Today's data is: ".

The following is a record containing fields named "OrderID", "CustomerID", "Item", and "Price" with values 1, 1, "Fishing rod", and 100.00. Square brace characters [ ] denote the beginning and end of a record expression, and are used to get a field value from a record. The following examples show a record and how to get the Item field value.

Here's an example record:

let Source =
        [
              OrderID = 1,
              CustomerID = 1,
              Item = "Fishing rod",
              Price = 100.00
        ]
in Source

To get the value of an Item, you use square brackets as Source[Item]:

let Source =
    [
          OrderID = 1,
          CustomerID = 1,
          Item = "Fishing rod",
          Price = 100.00
    ]
in Source[Item] //equals "Fishing rod"

Table

A Table is a set of values organized into named columns and rows. The column type can be implicit or explicit. You can use #table to create a list of column names and list of rows. A Table of values is a List in a List. The curly brace characters { } are also used to retrieve a row from a Table by index position (go to Example 3 - Get a row from a table by index position).

Example 1 - Create a table with implicit column types

let
  Source = #table(
    {"OrderID", "CustomerID", "Item", "Price"},
      {
          {1, 1, "Fishing rod", 100.00},
          {2, 1, "1 lb. worms", 5.00}
      })
in
    Source

Example 2 - Create a table with explicit column types

let
    Source = #table(
    type table [OrderID = number, CustomerID = number, Item = text, Price = number],
        {
                {1, 1, "Fishing rod", 100.00},
             {2, 1, "1 lb. worms", 5.00}
        }
    )
in
    Source

Both of the previous examples creates a table with the following shape:

OrderID CustomerID Item Price
1 1 Fishing rod 100.00
2 1 1 lb. worms 5.00

Example 3 - Get a row from a table by index position

let
    Source = #table(
    type table [OrderID = number, CustomerID = number, Item = text, Price = number],
        {
              {1, 1, "Fishing rod", 100.00},
              {2, 1, "1 lb. worms", 5.00}
         }
    )
in
    Source{1}

This expression returns the follow record:

Field Value
OrderID 2
CustomerID 1
Item 1 lb. worms
Price 5

Additional structured data examples

Structured data can contain any M value. Here are some examples:

Example 1 - List with [Primitive](#_Primitive_value_1) values, [Function](#_Function_value), and [Record](#_Record_value)

let
    Source =
{
   1,
   "Bob",
   DateTime.ToText(DateTime.LocalNow(), "yyyy-MM-dd"),
   [OrderID = 1, CustomerID = 1, Item = "Fishing rod", Price = 100.0]
}
in
    Source

Evaluating this expression can be visualized as:

Diagram of a table that visualizes the expression with a primitive value, a function, and a record.

Example 2 - Record containing primitive values and nested records

let
    Source = [CustomerID = 1, Name = "Bob", Phone = "123-4567", Orders =
        {
              [OrderID = 1, CustomerID = 1, Item = "Fishing rod", Price = 100.0],
            [OrderID = 2, CustomerID = 1, Item = "1 lb. worms", Price = 5.0]
        }]
in
    Source

Evaluating this expression can be visualized as:

Diagram of a table that visualizes the expression with several primitive values and nested records.

Note

Although many values can be written literally as an expression, a value isn't an expression. For example, the expression 1 evaluates to the value 1; the expression 1+1 evaluates to the value 2. This distinction is subtle, but important. Expressions are recipes for evaluation; values are the results of evaluation.

If expression

The if expression selects between two expressions based on a logical condition. For example:

if 2 > 1 then
    2 + 2
else
    1 + 1

The first expression (2 + 2) is selected if the logical expression (2 > 1) is true, and the second expression (1 + 1) is selected if it is false. The selected expression (in this case 2 + 2) is evaluated and becomes the result of the if expression (4).