The Power Query M formula language is optimized for building highly flexible data mashup queries. It's a functional, case sensitive language similar to F#. Power Query Formula Language is used in a number of Microsoft products such as Power BI Desktop, Excel, and Analysis Services.

Power Query M language specification

The specification describes the values, expressions, environments and variables, identifiers, and the evaluation model that form the Power Query M language’s basic concepts.

​Power Query M language specification.pdf​

Power Query M type system

The Types in Power Query M formula language document describes the M type system.

​Types in Power Query M formula language .pdf​