Power Query Decompiler

A proof of concept for decompiling Power Query (M) functions back into readable M source code, by walking the undocumented AST returned by Value.ResourceExpression.

Usage

Paste Decompile.pq into Power BI Desktop / Excel Power Query as a query named Decompile, then invoke it on any decompilable function:

= Decompile(List.Range)

which produces:

(list as list, offset as number, optional count as number) as list =>
let
    skippedInput = List.RemoveFirstN(list, offset),
    t115 = if count = null then skippedInput else List.FirstN(skippedInput, count)
in
    t115

What it handles

• Stripping the compiler's runtime type-checking harness (Value.As wrappers and immediate lambda self-applications), while harvesting the type information it carries to re-emit as ascriptions on function signatures
• Re-sugaring desugared let expressions (the compiler turns let into record field access), including recursive bindings (@f)
• if/then/else, binary and unary operators, a .. b ranges, x meta y
• error, try ... otherwise, try ... catch
• Lists, records, field/element access, each lambdas, optional parameters
• Resolving references to other library functions through the closure's constant pool

Known limitations: non-primitive type expressions are only recovered down to their primitive kind, genuinely anonymous helper functions render as function, enum constants render as their raw numbers (e.g. 1200 for TextEncoding.Unicode), and there is no round-trip verification yet.

Structure

File	Role
Decompile.pq	The decompiler as a function (fn as function) => text. Single source of truth for all logic.
Expression.pq	Same logic exposed as a query, with the target function bound inline (fn = List.Range).
AST2json.pq	Diagnostic: serializes the raw AST from Value.ResourceExpression to JSON — the primary debugging tool: Text.FromBinary(AST2json(fn)).
AST.pq	Historical demo of a raw path into the AST for List.Range (superseded by Decompile.pq).
Test.pq	Regression harness: runs Decompile over every decompilable function in #shared and reports a Name / Ok / Code / Error table. Requires a query named Decompile in the same workbook.

AST.pq demos the original deep call into the AST:

=Value.ResourceExpression(List.Range)
    [Expression]
        [Arguments]{0}
            [Function]
                [Expression]
                    [Expression]

Functions that are "decompilable"

Decompilation works for functions whose resource expression has [Kind] = "Function" — roughly 100 functions from the standard library, plus user-defined functions whose resource expression is exposed. The query that returns the list is:

= List.Select(Record.FieldNames(#shared),each Value.ResourceExpression(Record.Field(#shared,_))[Kind]="Function")

Which brings up this output:

Binary.View , Cdm.MapToEntity , Date.DayOfWeekName , Date.IsInCurrentDay , Date.IsInCurrentMonth , Date.IsInCurrentQuarter , Date.IsInCurrentWeek , Date.IsInCurrentYear , Date.IsInNextDay , Date.IsInNextMonth , Date.IsInNextNDays , Date.IsInNextNMonths , Date.IsInNextNQuarters , Date.IsInNextNWeeks , Date.IsInNextNYears , Date.IsInNextQuarter , Date.IsInNextWeek , Date.IsInNextYear , Date.IsInPreviousDay , Date.IsInPreviousMonth , Date.IsInPreviousNDays , Date.IsInPreviousNMonths , Date.IsInPreviousNQuarters , Date.IsInPreviousNWeeks , Date.IsInPreviousNYears , Date.IsInPreviousQuarter , Date.IsInPreviousWeek , Date.IsInPreviousYear , Date.IsInYearToDate , Date.MonthName , DateTime.IsInCurrentHour , DateTime.IsInCurrentMinute , DateTime.IsInCurrentSecond , DateTime.IsInNextHour , DateTime.IsInNextMinute , DateTime.IsInNextNHours , DateTime.IsInNextNMinutes , DateTime.IsInNextNSeconds , DateTime.IsInNextSecond , DateTime.IsInPreviousHour , DateTime.IsInPreviousMinute , DateTime.IsInPreviousNHours , DateTime.IsInPreviousNMinutes , DateTime.IsInPreviousNSeconds , DateTime.IsInPreviousSecond , Emigo.GetExtractFunction , EmigoDataSourceConnector.GetExtractFunction , HexagonSmartApi.ApplySelectList , HexagonSmartApi.ExecuteParametricFilterOnFilterRecord , HexagonSmartApi.ExecuteParametricFilterOnFilterUrl , HexagonSmartApi.GenerateParametricFilterByFilterSourceType , HexagonSmartApi.Typecast , List.FindText , List.MatchesAll , List.MatchesAny , List.NonNullCount , List.Range , List.RemoveItems , List.RemoveLastN , List.ReplaceValue , MongoDBAtlasODBC.Query , Replacer.ReplaceValue , SqlExpression.SchemaFrom , Table.AddColumn , Table.AddRankColumn , Table.AlternateRows , Table.ColumnCount , Table.ColumnsOfType , Table.CombineColumns , Table.CombineColumnsToRecord , Table.Contains , Table.ContainsAll , Table.ContainsAny , Table.DemoteHeaders , Table.DuplicateColumn , Table.ExpandListColumn , Table.ExpandTableColumn , Table.FillUp , Table.FindText , Table.FirstValue , Table.HasColumns , Table.InsertRows , Table.IsDistinct , Table.IsEmpty , Table.Last , Table.LastN , Table.MatchesAllRows , Table.MatchesAnyRows , Table.Max , Table.MaxN , Table.Min , Table.MinN , Table.Partition , Table.PositionOf , Table.PositionOfAny , Table.PrefixColumns , Table.Profile , Table.Range , Table.RemoveLastN , Table.RemoveMatchingRows , Table.RemoveRows , Table.Repeat , Table.ReplaceMatchingRows , Table.ReplaceRows , Table.ReplaceValue , Table.ReverseRows , Table.Schema , Table.SplitColumn , Table.ToColumns , Table.ToRows , Table.TransformRows , Table.Transpose , Table.View , Text.AfterDelimiter , Text.BeforeDelimiter , Text.BetweenDelimiters , Type.TableSchema

Caveats

Value.ResourceExpression is undocumented and the AST shapes it returns may change between engine versions without notice — the decompiler is written defensively, but expect drift. When it encounters an AST node kind it does not know, it fails loudly with a Decompile.UnknownKind error carrying the raw node.