Power Query M formula language: let expression

Arguably, the most simple let expression is

let in 42

When evaluated in Excel, for example with this VBA code, this formula evaluates to:

It's possible to define variables in between the let and the in keywords. The value of these variables can then be used in the part after the in.

let
   variable_1      = 18,
   unused_variable = 99,
   variable_2      = 24
in
   variable_1 + variable_2

This code produces the same result as the «most simple let expression» above.

When a let expression is evaluated, the dependencies between variables in the variable list are resolved automatically. In the following example, bar is assigned first, then baz followed by foo calculated, resulting in 42.

let
   foo = 2 * baz,
   bar = 7,
   baz = 3 * bar
in
   foo

Compare with automatic resolving of dependencies in records.

let expressions can be nested. The following expression results in 6:

let
   one   = let
              a = -2,
              b =  3
           in
              a + b,

   two   = let
              c =  5,
              d =  3
           in
              c - d,

   three = let
              e = 1
           in
              e + two

in
   one + two + three