Add lambda expression and `map()` + `filter()` functions with ARM syntax by Gijsreyn · Pull Request #1238 · PowerShell/DSC

Gijsreyn · 2025-11-05T03:15:53Z

PR Summary

This PR implements lambda expressions and the map() function for DSC configuration documents, enabling array transformations. The implementation uses ARM compatible syntax e.g., (lambda('param', body) and lambdaVariables('param'))

Example usage:

# Transform array elements
map(createArray(1, 2, 3), lambda('x', mul(lambdaVariables('x'), 2)))
# Returns: [2, 4, 6]

Seperated tests in a different file.

PR Context

Partially addresses #57.

Copilot

Pull Request Overview

This PR implements lambda expressions and the map() function for DSC configuration documents, using ARM-compatible syntax (lambda('param', body) and lambdaVariables('param')). The implementation stores lambdas in the context with unique UUID-based identifiers and evaluates them by binding lambda parameters to array elements.

Key changes:

Added Lambda struct and lambda-specific handling in the function evaluation pipeline
Implemented map(), lambda(), and lambdaVariables() functions for array transformations
Extended Context with lambda storage (lambdas) and lambda parameter bindings (lambda_variables)

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
lib/dsc-lib/src/parser/functions.rs	Added `Lambda` struct and `FunctionArg::Lambda` variant; special handling for lambda() function to bypass normal evaluation
lib/dsc-lib/src/functions/mod.rs	Implemented `invoke_lambda()` to create and store lambda objects with UUID-based IDs; registered new lambda-related functions
lib/dsc-lib/src/functions/map.rs	Implemented array transformation function that evaluates lambdas for each element with optional index parameter
lib/dsc-lib/src/functions/lambda_variables.rs	Implemented parameter lookup function for accessing lambda-bound variables during evaluation
lib/dsc-lib/src/functions/lambda.rs	Created lambda function stub that returns error if invoked directly (handled specially elsewhere)
lib/dsc-lib/src/configure/context.rs	Added `lambda_variables` HashMap and `lambdas` RefCell for lambda storage and parameter binding
lib/dsc-lib/locales/en-us.toml	Added localized error messages and descriptions for lambda-related functions
dsc/tests/dsc_lambda.tests.ps1	Added integration tests for map() with various lambda scenarios (multiplication, index usage, range, empty array)

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lib/dsc-lib/src/configure/context.rs

lib/dsc-lib/src/parser/functions.rs

lib/dsc-lib/src/functions/mod.rs

lib/dsc-lib/src/functions/map.rs

dsc/tests/dsc_lambda.tests.ps1

lib/dsc-lib/src/parser/functions.rs

…into PowerShellgh-57/main/add-map-lambda-function

…//github.com/Gijsreyn/operation-methods into PowerShellgh-57/main/add-map-lambda-function

lib/dsc-lib/src/functions/map.rs

lib/dsc-lib/src/functions/lambda_variables.rs

lib/dsc-lib/src/functions/map.rs

lib/dsc-lib/src/functions/mod.rs

SteveL-MSFT · 2025-12-16T23:48:21Z

lib/dsc-lib/src/functions/map.rs

+        for (index, element) in array.iter().enumerate() {
+            // Create a new context with lambda variables bound
+            let mut lambda_context = context.clone();
+
+            // Bind first parameter to array element
+            lambda_context.lambda_variables.insert(
+                lambda.parameters[0].clone(),
+                element.clone()
+            );
+
+            // Bind second parameter to index if provided
+            if lambda.parameters.len() == 2 {
+                lambda_context.lambda_variables.insert(
+                    lambda.parameters[1].clone(),
+                    Value::Number(serde_json::Number::from(index))
+                );
+            }
+
+            // Evaluate lambda body with bound variables
+            let result = lambda.body.invoke(&dispatcher, &lambda_context)?;
+            result_array.push(result);
+        }


I haven't thought this through, but instead of having each lambda function implement processing lambdas, it would seem like this could be in lambda() itself so the other lambda functions in https://learn.microsoft.com/en-us/azure/azure-resource-manager/templates/template-functions-lambda would just handle how to apply the lambda result.

Perhaps it would help to also implement filter() in this PR to see how it would work?

I added the filter() function to compare, where I had introduced a new file (lamba_helpers.rs). Mind checking again?

…data

SteveL-MSFT

LGTM

SteveL-MSFT · 2026-03-02T21:49:35Z

lib/dsc-lib/src/functions/lambda_variables.rs

+                FunctionArgKind::String,
+                FunctionArgKind::Number,
+                FunctionArgKind::Boolean,
+                FunctionArgKind::Array,
+                FunctionArgKind::Object,
+                FunctionArgKind::Null,


Suggested change

FunctionArgKind::String,

FunctionArgKind::Number,

FunctionArgKind::Boolean,

FunctionArgKind::Array,

FunctionArgKind::Object,

FunctionArgKind::Null,

FunctionArgKind::Array,

FunctionArgKind::Boolean,

FunctionArgKind::Null,

FunctionArgKind::Number,

FunctionArgKind::Object,

FunctionArgKind::String,

SteveL-MSFT · 2026-03-02T21:50:41Z

@Gijsreyn can you resolve the merge conflict and accept my suggestion to order then alpha?

Gijsreyn added 3 commits November 5, 2025 02:05

Initial setup

dd9d3d3

Fix comments

fed30d4

Re-add localization

9b05728

Gijsreyn marked this pull request as ready for review November 5, 2025 03:30

SteveL-MSFT requested a review from Copilot November 12, 2025 22:33

Copilot started reviewing on behalf of SteveL-MSFT November 12, 2025 22:34 View session

Copilot finished reviewing on behalf of SteveL-MSFT November 12, 2025 22:37

Copilot AI reviewed Nov 12, 2025

View reviewed changes

Gijsreyn added 5 commits November 14, 2025 02:14

Merge branch 'main' of https://github.com/Gijsreyn/operation-methods …

97cf256

…into PowerShellgh-57/main/add-map-lambda-function

Remove dead code and add instructions

acc8181

Initial setup

3e019fe

Fix comments

a81882c

Re-add localization

b180986

Gijsreyn force-pushed the gh-57/main/add-map-lambda-function branch from 97cf256 to b180986 Compare November 14, 2025 01:15

Gijsreyn added 6 commits November 14, 2025 02:16

Merge branch 'PowerShellgh-57/main/add-map-lambda-function' of https:…

fc04a3b

…//github.com/Gijsreyn/operation-methods into PowerShellgh-57/main/add-map-lambda-function

Merge branch 'main' into PowerShellgh-57/main/add-map-lambda-function

00f9701

Remove conflict

3176d5b

Fix test to look at output

a93f9e2

Merge branch 'main' into PowerShellgh-57/main/add-map-lambda-function

3139835

Add lambda expression support for DSC function expressions

0869cf0

SteveL-MSFT requested changes Dec 2, 2025

View reviewed changes

lib/dsc-lib/src/functions/map.rs Outdated Show resolved Hide resolved

Gijsreyn added 2 commits December 12, 2025 03:43

Merge branch 'main' into PowerShellgh-57/main/add-map-lambda-function

4d831b1

Add support for Lambda function arguments in the function dispatcher

fb353f4

Gijsreyn requested a review from SteveL-MSFT December 12, 2025 05:39

SteveL-MSFT requested changes Dec 12, 2025

View reviewed changes

Gijsreyn added 5 commits December 13, 2025 03:54

Merge branch 'main' into PowerShellgh-57/main/add-map-lambda-function

e9df293

Refactor argument handling

af25cc2

Remove test

7a1de83

Remove unused localization strings

9057146

Add Lambda process mode and update function invocation logic

7c3c6b6

Merge branch 'main' into PowerShellgh-57/main/add-map-lambda-function

f5a902d

Gijsreyn requested a review from SteveL-MSFT December 16, 2025 02:12

SteveL-MSFT requested changes Dec 16, 2025

View reviewed changes

Implement filter function with lambda support and update related meta…

c3d6d77

…data

Gijsreyn changed the title ~~Add lambda expression and map() function with ARM syntax~~ Add lambda expression and map() + filter() functions with ARM syntax Dec 17, 2025

Gijsreyn requested a review from SteveL-MSFT December 17, 2025 02:48

SteveL-MSFT approved these changes Mar 2, 2026

View reviewed changes

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Gijsreyn commented Nov 5, 2025

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