Testing & Error Reporting

Running Tests

All language and error tests live in tests/ and run via the TestDang suite:

# Run all tests
go test ./tests/ -v

# Run a specific test by name
go test ./tests/ -run "TestDang/TestLanguage/test_foo" -v
go test ./tests/ -run "TestDang/TestErrorMessages/some_error" -v
go test ./tests/ -run "TestDang/TestFormatLanguage/test_foo" -v

Test Suites

The DangSuite (in tests/integration_test.go) has three sub-suites:

• •TestLanguage: Runs each tests/test_*.dang file and expects it to succeed.
• •TestFormatLanguage: Formats each tests/test_*.dang file, re-parses the formatted output, runs it, and checks it still produces the same result.
• •TestErrorMessages: Runs each tests/errors/*.dang file, expects an error, and compares the output against tests/testdata/<name>.golden.

Error Message Golden Files

Error golden files live in tests/testdata/ and contain the exact error output including ANSI escape codes for colored/highlighted source annotations.

To update golden files after changing error messages:

go test ./tests/ -run "TestDang/TestErrorMessages" -update

Adding a New Error Test

1. •Create tests/errors/my_error.dang with code that should produce an error.
2. •Run with -update to generate the golden file:
   bash

   go test ./tests/ -run "TestDang/TestErrorMessages/my_error" -update
   

3. •Review tests/testdata/my_error.golden to confirm the error message is correct.

Dagger SDK Tests

The Dagger SDK has its own integration test suite in dagger-sdk/tests/ that exercises modules against a real Dagger engine via dagger call.

Running SDK Tests

# Run all SDK tests
go test ./dagger-sdk/tests/ -v

# Run a specific test
go test ./dagger-sdk/tests/ -run "TestDaggerSDK/TestMismatch" -v

Test Structure

• •Test suite: DaggerSDKSuite in dagger-sdk/tests/integration_test.go
• •Test modules: Each test has a Dagger module in dagger-sdk/testdata/<module-name>/ containing:
  • •main.dang — the module source
  • •dagger.json — module config with "sdk": "../.." pointing to the SDK

Adding a New SDK Test

1. •Create dagger-sdk/testdata/my-test/main.dang with a Dagger module.
2. •Create dagger-sdk/testdata/my-test/dagger.json:
   json

   {
     "name": "my-test",
     "engineVersion": "v0.19.11",
     "sdk": "../.."
   }
   

3. •Add a test method on DaggerSDKSuite in dagger-sdk/tests/integration_test.go:
   go

   func (DaggerSDKSuite) TestMyTest(ctx context.Context, t *testctx.T) {
       t.Run("sub test", func(ctx context.Context, t *testctx.T) {
           out := requireDagger(ctx, t, "my-test", "some-function", "--arg", "value")
           require.Contains(t, out, "expected output")
       })
   }
   

Helpers

• •runDagger(ctx, module, args...) — runs dagger -m <testdata/module> call <args>, returns stdout, stderr, error
• •requireDagger(ctx, t, module, args...) — same but fails the test on error, returns stdout

Reporting Nice Errors

Dang has two error types for attaching source locations:

InferError (type checking phase)

Use NewInferError in Infer() methods to attach source location to type errors. These get converted to SourceError with full source highlighting when displayed.

return nil, NewInferError(fmt.Errorf("descriptive message"), node)

WrapInferError avoids double-wrapping if the error is already an InferError:

return nil, WrapInferError(err, node)

SourceError (display/eval phase)

SourceError holds the error, source location, and source text, and renders with highlighted code snippets:

return nil, NewSourceError(fmt.Errorf("message"), location, sourceCode)

During eval, use the EvalContext helper:

return nil, evalCtx.CreateSourceError(err, node)

Conversion Flow

InferError → ConvertInferError() → SourceError (reads the source file and produces highlighted output). This happens automatically in RunFile.

WithInferErrorHandling / WithEvalErrorHandling

Wrapper helpers that automatically attach source location to errors that don't already have one:

func (n *MyNode) Infer(ctx context.Context, env hm.Env, fresh hm.Fresher) (hm.Type, error) {
    return WithInferErrorHandling(n, func() (hm.Type, error) {
        // ... your logic here
    })
}

PEG Grammar

The parser is defined in pkg/dang/dang.peg (pigeon PEG parser). After editing:

go generate ./pkg/dang/

This regenerates pkg/dang/dang.peg.go.

Formatter

The formatter lives in pkg/dang/format.go. When adding a new AST node type, update:

• •formatNode() — add a case to format the node
• •nodeLocation() — return the node's *SourceLocation
• •Any relevant classification helpers (e.g. isFunctionDef) for blank line insertion logic