Disallow user constraints from returning custom error messages.

CHANGELOG.md

@@ -22,6 +22,7 @@ The format is based on [Keep a Changelog], and this project adheres to
 
 - **[BC]** Floating point variables no longer accepted `NaN`, `+Inf` or `-Inf` as valid values
 - **[BC]** `variable.Register()` now directly accepts a `Registry` instead of using functional options
+- **[BC]** Replaced `WithConstraintFunc()` with `WithConstraint()`
 
 ### Removed
 

modevalidateexample_test.go

@@ -1,13 +1,11 @@
 package ferrite_test
 
 import (
-	"errors"
 	"os"
 	"strings"
 	"time"
 
 	"github.com/dogmatiq/ferrite"
-	"github.com/dogmatiq/ferrite/variable"
 )
 
 func ExampleInit_validation() {
@@ -323,27 +321,21 @@ func ExampleInit_validationWithInvalidValues() {
 	os.Setenv("FERRITE_STRING", "foo bar")
 	ferrite.
 		String("FERRITE_STRING", "example string").
-		WithConstraintFunc(
-			"MUST NOT contain whitespace",
-			func(s string) variable.ConstraintError {
-				if strings.ContainsRune(s, ' ') {
-					return errors.New("must not contain whitespace")
-				}
-				return nil
+		WithConstraint(
+			"must not contain whitespace",
+			func(s string) bool {
+				return !strings.ContainsRune(s, ' ')
 			},
 		).
 		Required()
 
 	os.Setenv("FERRITE_STRING_SENSITIVE", "foo bar")
 	ferrite.
 		String("FERRITE_STRING_SENSITIVE", "example sensitive string").
-		WithConstraintFunc(
-			"MUST NOT contain whitespace",
-			func(s string) variable.ConstraintError {
-				if strings.ContainsRune(s, ' ') {
-					return errors.New("must not contain whitespace")
-				}
-				return nil
+		WithConstraint(
+			"must not contain whitespace",
+			func(s string) bool {
+				return !strings.ContainsRune(s, ' ')
 			},
 		).
 		WithSensitiveContent().

numfloat.go

@@ -25,7 +25,7 @@ func Float[T constraints.Float](name, desc string) *FloatBuilder[T] {
 	b.spec.Name(name)
 	b.spec.Description(desc)
 	b.spec.BuiltInConstraint(
-		"MUST be a fine number",
+		"must be a finite number",
 		func(v T) variable.ConstraintError {
 			v64 := float64(v)
 			if math.IsNaN(v64) || math.IsInf(v64, 0) {

string.go

@@ -42,16 +42,13 @@ func (b *StringBuilder[T]) WithDefault(v T) *StringBuilder[T] {
 	return b
 }
 
-// WithConstraintFunc adds a constraint function to the variable.
+// WithConstraint adds a constraint to the variable.
 //
 // fn is called with the environment variable value after it is parsed. If fn
-// returns an error the value is considered invalid.
-//
-// Care should be taken never to include the value in the error message, as it
-// may contain sensitive information.
-func (b *StringBuilder[T]) WithConstraintFunc(
+// returns false the value is considered invalid.
+func (b *StringBuilder[T]) WithConstraint(
 	desc string,
-	fn func(T) variable.ConstraintError,
+	fn func(T) bool,
 ) *StringBuilder[T] {
 	b.spec.UserConstraint(desc, fn)
 	return b

stringexample_test.go

@@ -1,12 +1,10 @@
 package ferrite_test
 
 import (
-	"errors"
 	"fmt"
 	"os"
 
 	"github.com/dogmatiq/ferrite"
-	"github.com/dogmatiq/ferrite/variable"
 )
 
 func ExampleString_required() {
@@ -66,10 +64,12 @@ func ExampleString_sensitive() {
 	os.Setenv("FERRITE_STRING", "hunter2")
 	ferrite.
 		String("FERRITE_STRING", "example sensitive string variable").
-		WithConstraintFunc(
+		WithConstraint(
 			"always fail",
-			func(s string) variable.ConstraintError {
-				return errors.New("always fail")
+			func(s string) bool {
+				// Force the variable to be considered invalid so that the
+				// variable table is rendered to the console.
+				return false
 			},
 		).
 		WithSensitiveContent().

variable/specbuilder.go

@@ -57,11 +57,21 @@ func (b *TypedSpecBuilder[T]) BuiltInConstraint(
 // UserConstraint adds a user-defined constraint to the variable's value.
 func (b *TypedSpecBuilder[T]) UserConstraint(
 	desc string,
-	fn func(T) ConstraintError,
+	fn func(T) bool,
 ) {
 	b.spec.constraints = append(
 		b.spec.constraints,
-		constraint[T]{desc, true, fn},
+		constraint[T]{
+			desc,
+			true,
+			func(v T) ConstraintError {
+				if fn(v) {
+					return nil
+				}
+
+				return errors.New(desc)
+			},
+		},
 	)
 }