Add lambda expression and `map()` function with ARM syntax #1238

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

SteveL-MSFT · 2025-12-12T23:03:12Z

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

+
+        if args.len() != 1 {
+            return Err(DscError::Parser(t!("functions.invalidArgCount", name = "lambdaVariables", count = 1).to_string()));
+        }


This isn't needed as function validation ensures min_args and max_args is correct

SteveL-MSFT · 2025-12-12T23:03:35Z

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

+        let Some(var_name) = args[0].as_str() else {
+            return Err(DscError::Parser(t!("functions.lambdaVariables.paramNameMustBeString").to_string()));
+        };


Same here, just unwrap.

SteveL-MSFT · 2025-12-12T23:04:44Z

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

+        if args.len() != 2 {
+            return Err(DscError::Parser(t!("functions.invalidArgCount", name = "map", count = 2).to_string()));
+        }
+
+        let Some(array) = args[0].as_array() else {
+            return Err(DscError::Parser(t!("functions.map.firstArgMustBeArray").to_string()));
+        };
+
+        let Some(lambda_id) = args[1].as_str() else {
+            return Err(DscError::Parser(t!("functions.map.secondArgMustBeLambda").to_string()));
+        };


Same as above, this is all validated already so these strings won't be used

SteveL-MSFT · 2025-12-12T23:08:20Z

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

+    pub fn invoke_lambda(&self, args: &Option<Vec<crate::parser::functions::FunctionArg>>, context: &Context) -> Result<Value, DscError> {
+        use crate::parser::functions::{FunctionArg, Lambda};
+        use uuid::Uuid;
+
+        let Some(args) = args else {
+            return Err(DscError::Parser(t!("functions.lambda.requiresArgs").to_string()));
+        };
+
+        if args.len() < 2 {
+            return Err(DscError::Parser(t!("functions.lambda.requiresParamAndBody").to_string()));
+        }
+
+        // All arguments except the last must be string values (parameter names)
+        let mut parameters = Vec::new();
+        for arg in args.iter().take(args.len() - 1) {
+            match arg {
+                FunctionArg::Value(Value::String(s)) => {
+                    parameters.push(s.clone());
+                },
+                _ => {
+                    return Err(DscError::Parser(t!("functions.lambda.paramsMustBeStrings").to_string()));
+                }
+            }
+        }
+
+        // Last argument is the body expression
+        let body_expr = match &args[args.len() - 1] {
+            FunctionArg::Expression(expr) => expr.clone(),
+            _ => {
+                return Err(DscError::Parser(t!("functions.lambda.bodyMustBeExpression").to_string()));
+            }
+        };
+
+        // Create Lambda and store in Context with unique ID
+        let lambda = Lambda {
+            parameters,
+            body: body_expr,
+        };
+
+        let lambda_id = format!("__lambda_{}", Uuid::new_v4());
+        context.lambdas.borrow_mut().insert(lambda_id.clone(), lambda);
+
+        // Return the ID as a string value
+        Ok(Value::String(lambda_id))
+    }


Instead of special handling it this way, you should have a new ProcessMode::Lambda and just have the lambda() function do this only when process mode is correct other return an error. This keeps it consistent with modes.

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

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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

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Gijsreyn commented Nov 5, 2025

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Add lambda expression and `map()` function with ARM syntax #1238

Add lambda expression and `map()` function with ARM syntax #1238