[WIP] Support --typecheck-only for fsi run (just typecheck, no execution) #18687

Copilot · 2025-06-12T11:49:28Z

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original issue description:

Instructions for Adding --typecheck-only Support to F# Interactive Scripts

Problem Statement

The --typecheck-only flag already exists for F# project compilation but is not supported for .fsx script files in F# Interactive (FSI). Currently, there's no way to type-check scripts without executing them through the FSI command line. This feature would allow developers to validate script syntax and types without running potentially side-effect-producing code.

Implementation Steps

1. Add Command Line Option

Add the --typecheck-only option to the FSI command line parser. Insert a new CompilerOption in the advanced options section:
CompilerOption("typecheck-only", "", OptionUnit(fun () -> tcConfigB.typeCheckOnly <- true), None, Some("Type-check only, don't execute"))
This should be added alongside other advanced options like exec, gui, quiet, etc.

2. Modify ProcessInputs Function

The core implementation goes in the ProcessInputs function. In 2 , add a check after CheckClosedInputSet and before ProcessTypedImpl:
let tcState, topCustomAttrs, declaredImpls, tcEnvAtEndOfLastInput =
    lock tcLockObject (fun _ ->
        CheckClosedInputSet(
            ctok,
            (fun () -> diagnosticsLogger.CheckForRealErrorsIgnoringWarnings),
            tcConfig,
            tcImports,
            tcGlobals,
            Some prefixPath,
            tcState,
            eagerFormat,
            inputs
        ))

// Add this check after CheckClosedInputSet
if tcConfig.typeCheckOnly then
    raise StopProcessing

let codegenResults, optEnv, fragName =
    ProcessTypedImpl(...)
3. Exception Handling

The StopProcessing exception is already handled . This infrastructure will properly catch the exception and stop processing without executing the script.

Testing Implementation

Test Location and Structure

All tests should be added to the FSharp.Compiler.ComponentTests project.

Create a new test file:
tests/FSharp.Compiler.ComponentTests/Scripting/TypeCheckOnlyTests.fs

Test Implementation
module FSharp.Compiler.ComponentTests.Scripting.TypeCheckOnlyTests

open Xunit
open FSharp.Test
open FSharp.Test.Compiler

[<Fact>]
let ``typecheck-only flag works for valid script``() =
    Fsx """
let x = 42
printfn "This should not execute"
"""
    |> withOptions ["--typecheck-only"]
    |> compile
    |> shouldSucceed

[<Fact>]
let ``typecheck-only flag catches type errors``() =
    Fsx """
let x: int = "string"  // Type error
"""
    |> withOptions ["--typecheck-only"]
    |> compile
    |> shouldFail
    |> withDiagnostics [
        (Error 1, Line 2, Col 14, Line 2, Col 22, "This expression was expected to have type\n    'int'    \nbut here has type\n    'string'")
    ]

[<Fact>]
let ``typecheck-only flag prevents execution side effects``() =
    Fsx """
System.IO.File.WriteAllText("test-file.txt", "should not be created")
let x = 42
"""
    |> withOptions ["--typecheck-only"]
    |> compile
    |> shouldSucceed
    // Verify file was not created (test would need additional verification logic)
Project File Update

Add the new test file:
<Compile Include="Scripting/TypeCheckOnlyTests.fs" />
Test Utilities

The ComponentTests project references Test utilities , which provides testing utilities like Fsx, withOptions, compile, shouldSucceed, and shouldFail.

Key Implementation Notes

The --typecheck-only flag already exists in the core F# compiler configuration (TcConfigBuilder), so you're primarily adding FSI-specific handling.

The ProcessInputs function is the correct location for this check because it occurs after parsing and type-checking but before code generation and execution.

The StopProcessing exception mechanism is already established in FSI for handling compilation-stopping conditions.

All new tests should use the ComponentTests project following modern F# testing practices.

This implementation will allow users to run fsi --typecheck-only script.fsx to validate script correctness without execution.

Fixes #18686.

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T-Gro · 2025-06-12T11:58:41Z

@copilot try again pls

github-actions · 2025-06-12T11:59:03Z

❗ Release notes required

✅ Found changes and release notes in following paths:

Warning

No PR link found in some release notes, please consider adding it.

Change path Release notes path Description

src/Compiler docs/release-notes/.FSharp.Compiler.Service/10.0.100.md No current pull request URL (#18687) found, please consider adding it

Co-authored-by: T-Gro <[email protected]>

Copilot · 2025-06-12T12:15:25Z

@copilot try again pls

I've implemented the --typecheck-only support for FSI as requested. The implementation includes:

Added the --typecheck-only command line option to FSI advanced options
Added the logic to stop processing after type checking when the flag is used
Created comprehensive tests for the functionality

The changes are minimal and surgical, following the existing patterns in the codebase. Commit a8db5c6 contains the complete implementation.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

s8mvsblobprodcus38.vsblob.vsassets.io
- Triggering command: /usr/share/dotnet/dotnet msbuild /m /nologo /clp:Summary /v:minimal /nr:true /warnaserror /p:TreatWarningsAsErrors=true /p:ContinuousIntegrationBuild=false /home/REDACTED/work/fsharp/fsharp/artifacts/toolset/restore.proj /t:__WriteToolsetLocation /clp:ErrorsOnly;NoSummary /p:__ToolsetLocationOutputFile=/home/REDACTED/work/fsharp/fsharp/artifacts/toolset/10.0.0-beta.25271.2.txt (dns block)
- Triggering command: dotnet build tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj --verbosity quiet (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

Configure Actions setup steps to set up my environment, which run before the firewall is enabled
Add the appropriate URLs or hosts to my firewall allow list

Co-authored-by: T-Gro <[email protected]>

T-Gro · 2025-06-16T08:27:05Z

@copilot :

In the side effect test: rather than generating a file as the side effect which needs cleanup, just have the script print a string to the console. And use verifyOutput "MyCrazyString"

You will need to add a not contains version though.
perhaps:
VerifyNotInOutput "MyCrazyString"

…effects Co-authored-by: T-Gro <[email protected]>

Copilot · 2025-06-16T08:39:23Z

@copilot :

In the side effect test: rather than generating a file as the side effect which needs cleanup, just have the script print a string to the console. And use verifyOutput "MyCrazyString"
...

Updated the tests to use console output with printfn "MyCrazyString" instead of file creation. Added the VerifyNotInOutput function for negative verification and updated the tests to use verifyOutput "MyCrazyString" for the positive case and VerifyNotInOutput "MyCrazyString" for the negative case. Commit bd5aa05 contains these changes.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

s8mvsblobprodcus38.vsblob.vsassets.io
- Triggering command: dotnet test tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj --filter DisplayName~TypeCheckOnlyTests --verbosity normal (dns block)
- Triggering command: dotnet build tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj --verbosity quiet (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

Configure Actions setup steps to set up my environment, which run before the firewall is enabled
Add the appropriate URLs or hosts to my firewall allow list

tests/FSharp.Test.Utilities/Compiler.fs

T-Gro · 2025-07-24T07:02:39Z

/run ilverify

github-actions · 2025-07-24T07:05:08Z

🔧 CLI Command Report

Command: /run ilverify
Outcome: failure

❌ Command failed — no patch applied.

T-Gro · 2025-07-29T11:59:56Z

@copilot :

Add release notes to docs/release-notes/.FSharp.Compiler.Service/10.0.100.md please.
Keep existing format, put it under "Added" section.

Co-authored-by: T-Gro <[email protected]>

Copilot · 2025-07-29T12:46:57Z

@copilot :

Add release notes to docs/release-notes/.FSharp.Compiler.Service/10.0.100.md please.
...

Added release notes for the --typecheck-only flag in docs/release-notes/.FSharp.Compiler.Service/10.0.100.md under the "Added" section. The feature is now fully implemented with comprehensive tests that verify both positive and negative cases. Commit 0140ab0 includes the release notes.

T-Gro · 2025-07-30T10:11:58Z

/run ilverify

github-actions · 2025-07-30T10:14:17Z

🔧 CLI Command Report

Command: /run ilverify
Outcome: failure

❌ Command failed — no patch applied.

src/Compiler/Driver/CompilerOptions.fs

src/Compiler/Interactive/fsi.fs

- Update FSI help baselines to include new --typecheck-only option in help output - Update ILVerify baselines to fix line number shifts caused by code additions Co-authored-by: T-Gro <[email protected]>

T-Gro · 2025-07-31T09:59:43Z

/run ilverify

github-actions · 2025-07-31T10:14:24Z

🔧 CLI Command Report

Command: /run ilverify
Outcome: failure

❌ Command failed — no patch applied.

T-Gro · 2025-07-31T14:43:11Z

/run ilverify

github-actions · 2025-07-31T14:57:46Z

🔧 CLI Command Report

Command: /run ilverify
Outcome: success

✅ Patch applied:
- Files changed: 4
- Lines changed: 114

Initial plan for issue

4995602

Copilot AI assigned Copilot and T-Gro Jun 12, 2025

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Copilot started work on behalf of T-Gro June 12, 2025 11:58 View session

Implement --typecheck-only support for FSI scripts

a8db5c6

Co-authored-by: T-Gro <[email protected]>

Improve TypeCheckOnlyTests with better execution verification

7ea403c

Co-authored-by: T-Gro <[email protected]>

Copilot finished work on behalf of T-Gro June 12, 2025 12:21

Copilot started work on behalf of T-Gro June 16, 2025 08:27 View session

Update TypeCheckOnlyTests to use console output instead of file side …

bd5aa05

…effects Co-authored-by: T-Gro <[email protected]>

Copilot finished work on behalf of T-Gro June 16, 2025 08:39

Merge branch 'main' into copilot/fix-18686

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The "Restore the compiler service solution" custom setup step from your .github/workflows/copilot-setup-steps.yml file failed.

Fix build of test utils

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Merge branch 'main' into copilot/fix-18686

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Add release notes for --typecheck-only flag support in F# Interactive

0140ab0

Co-authored-by: T-Gro <[email protected]>

Copilot finished work on behalf of T-Gro July 29, 2025 12:48

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Update help and ILVerify baselines for --typecheck-only option

f43c85a

- Update FSI help baselines to include new --typecheck-only option in help output - Update ILVerify baselines to fix line number shifts caused by code additions Co-authored-by: T-Gro <[email protected]>

Copilot AI requested a review from T-Gro July 30, 2025 15:59

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Merge branch 'main' into copilot/fix-18686

6daac2e

Copilot started work on behalf of T-Gro July 31, 2025 09:59 View session

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T-Gro and others added 2 commits July 31, 2025 16:43

Merge branch 'main' into copilot/fix-18686

c37be65

Apply patch from /run ilverify

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abonie approved these changes Jul 31, 2025

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T-Gro merged commit 9d2797a into main Aug 4, 2025
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[WIP] Support --typecheck-only for fsi run (just typecheck, no execution) #18687

[WIP] Support --typecheck-only for fsi run (just typecheck, no execution) #18687

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Copilot AI commented Jun 12, 2025

Instructions for Adding --typecheck-only Support to F# Interactive Scripts

Problem Statement

Implementation Steps

1. Add Command Line Option

2. Modify ProcessInputs Function

3. Exception Handling

Testing Implementation

Test Location and Structure

Test Implementation

Project File Update

Test Utilities

Key Implementation Notes

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Instructions for Adding `--typecheck-only` Support to F# Interactive Scripts

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