Chapter 06: Check Collection Builder

Description

A var-level helper function wraps check functions into a slice: var checkXxx = func(fns ...checkXxxFn) []checkXxxFn { return fns }. This gives callers a clean, expressive API: checks: checkValidate(checkValid(true), checkErrorCount(1)) instead of the noisier checks: []checkValidateFn{checkValid(true), checkErrorCount(1)}.

Real-world examples:

• pantry/internal/core/domain/product_test.go:91 — var checkProductApplyMovement
• notifier/model/test_utils.go:13-19 — CheckNotifier, CheckResult
• jokes/internal/adapters/secondary/external/joke_client_test.go:13 — checkNewJokeClient

Code

type ValidationReport struct {
    Valid    bool
    Errors   []string
    Warnings []string
}

func ValidateDocument(doc *Document) *ValidationReport {
    // validates document, returns report with errors/warnings
}

Test

type checkValidateFn func(*testing.T, *ValidationReport)

var checkValidate = func(fns ...checkValidateFn) []checkValidateFn { return fns }

func TestValidateDocument(t *testing.T) {
    checkValid := func(want bool) checkValidateFn {
        return func(t *testing.T, r *ValidationReport) {
            t.Helper()
            assert.Equal(t, want, r.Valid)
        }
    }

    checkErrorCount := func(want int) checkValidateFn {
        return func(t *testing.T, r *ValidationReport) {
            t.Helper()
            assert.Len(t, r.Errors, want)
        }
    }

    tests := []struct {
        name   string
        doc    *Document
        checks []checkValidateFn
    }{
        {
            name: "valid document",
            doc:  &Document{Title: "Go Tips", Content: "Use interfaces to decouple code."},
            checks: checkValidate(        // <--- builder call, no [] type wrapper
                checkValid(true),
                checkErrorCount(0),
            ),
        },
        {
            name: "missing title",
            doc:  &Document{Title: "", Content: "Some content here."},
            checks: checkValidate(
                checkValid(false),
                checkErrorCount(1),
            ),
        },
    }
    // ...
}

Testing Approach

The check collection builder:

1. Cleaner syntax — checkValidate(checkValid(true)) reads better than []checkValidateFn{checkValid(true)}. The varargs builder eliminates the slice literal wrapper.
2. Zero-value consistency — checkValidate() returns an empty slice, meaning "no assertions to run." This is useful for placeholder/TODO test cases that need to compile but aren't filled in yet.
3. Builder naming convention — the variable is named after the check type without Fn suffix: checkValidate builds checkValidateFn. This creates a natural naming: checkXxx returns []checkXxxFn.
4. Composable with t.Helper() — all inner check factories call t.Helper(), so the stack trace points to the test case, not the helper. The builder itself (var checkValidate) is trivially simple and doesn't need t.Helper().

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View source code on GitHub