WithTransform() supports transform function with explicit types

matchers.go

diff --git a/matchers.go b/matchers.go
index 872de9e..ba41103 100644
--- a/matchers.go
+++ b/matchers.go
@@ -371,13 +371,11 @@
 }
 
 //WithTransform applies the `transform` to the actual value and matches it against `matcher`.
-//  var plus1 = func(i interface{}) interface{} { return i.(int) + 1 }
+//The given transform must be a function of one parameter that returns one value.
+//  var plus1 = func(i int) int { return i + 1 }
 //  Expect(1).To(WithTransform(plus1, Equal(2))
 //
 //And(), Or(), Not() and WithTransform() allow matchers to be composed into complex expressions.
-func WithTransform(transform func(interface{}) interface{}, matcher types.GomegaMatcher) types.GomegaMatcher {
-	return &matchers.WithTransformMatcher{
-		Transform: transform,
-		Matcher:   matcher,
-	}
+func WithTransform(transform interface{}, matcher types.GomegaMatcher) types.GomegaMatcher {
+	return matchers.NewWithTransformMatcher(transform, matcher)
 }

matchers/with_transform.go

diff --git a/matchers/with_transform.go b/matchers/with_transform.go
index c7a650a..430223e 100644
--- a/matchers/with_transform.go
+++ b/matchers/with_transform.go
@@ -1,25 +1,61 @@
 package matchers
 
-import "github.com/onsi/gomega/types"
+import (
+	"fmt"
+	"github.com/onsi/gomega/types"
+	"reflect"
+)
 
 type WithTransformMatcher struct {
 	// input
-	Transform func(interface{}) interface{}
+	Transform interface{} // must be a function of one parameter that returns one value
 	Matcher   types.GomegaMatcher
 
+	// cached value
+	transformArgType reflect.Type
+
 	// state
 	transformedValue interface{}
 }
 
+func NewWithTransformMatcher(transform interface{}, matcher types.GomegaMatcher) *WithTransformMatcher {
+	if transform == nil {
+		panic("transform function cannot be nil")
+	}
+	txType := reflect.TypeOf(transform)
+	if txType.NumIn() != 1 {
+		panic("transform function must have 1 argument")
+	}
+	if txType.NumOut() != 1 {
+		panic("transform function must have 1 return value")
+	}
+
+	return &WithTransformMatcher{
+		Transform:        transform,
+		Matcher:          matcher,
+		transformArgType: reflect.TypeOf(transform).In(0),
+	}
+}
+
 func (m *WithTransformMatcher) Match(actual interface{}) (bool, error) {
-	m.transformedValue = m.Transform(actual)
+	// return error if actual's type is incompatible with Transform function's argument type
+	actualType := reflect.TypeOf(actual)
+	if !actualType.AssignableTo(m.transformArgType) {
+		return false, fmt.Errorf("Transform function expects '%s' but we have '%s'", m.transformArgType, actualType)
+	}
+
+	// call the Transform function with `actual`
+	fn := reflect.ValueOf(m.Transform)
+	result := fn.Call([]reflect.Value{reflect.ValueOf(actual)})
+	m.transformedValue = result[0].Interface() // expect exactly one value
+
 	return m.Matcher.Match(m.transformedValue)
 }
 
-func (m *WithTransformMatcher) FailureMessage(actual interface{}) (message string) {
+func (m *WithTransformMatcher) FailureMessage(_ interface{}) (message string) {
 	return m.Matcher.FailureMessage(m.transformedValue)
 }
 
-func (m *WithTransformMatcher) NegatedFailureMessage(actual interface{}) (message string) {
+func (m *WithTransformMatcher) NegatedFailureMessage(_ interface{}) (message string) {
 	return m.Matcher.NegatedFailureMessage(m.transformedValue)
 }

matchers/with_transform_test.go

diff --git a/matchers/with_transform_test.go b/matchers/with_transform_test.go
index ced53bd..4d1a363 100644
--- a/matchers/with_transform_test.go
+++ b/matchers/with_transform_test.go
@@ -1,18 +1,56 @@
 package matchers_test
 
 import (
+	"errors"
 	. "github.com/onsi/ginkgo"
 	. "github.com/onsi/gomega"
 )
 
 var _ = Describe("WithTransformMatcher", func() {
 
-	var plus1 = func(i interface{}) interface{} { return i.(int) + 1 }
+	var plus1 = func(i int) int { return i + 1 }
+
+	Context("Panic if transform function invalid", func() {
+		panicsWithTransformer := func(transform interface{}) {
+			ExpectWithOffset(1, func() { WithTransform(transform, nil) }).To(Panic())
+		}
+		It("nil", func() {
+			panicsWithTransformer(nil)
+		})
+		Context("Invalid number of args, but correct return value count", func() {
+			It("zero", func() {
+				panicsWithTransformer(func() int { return 5 })
+			})
+			It("two", func() {
+				panicsWithTransformer(func(i, j int) int { return 5 })
+			})
+		})
+		Context("Invalid number of return values, but correct number of arguments", func() {
+			It("zero", func() {
+				panicsWithTransformer(func(i int) {})
+			})
+			It("two", func() {
+				panicsWithTransformer(func(i int) (int, int) { return 5, 6 })
+			})
+		})
+	})
 
 	It("works with positive cases", func() {
 		Expect(1).To(WithTransform(plus1, Equal(2)))
 		Expect(1).To(WithTransform(plus1, WithTransform(plus1, Equal(3))))
 		Expect(1).To(WithTransform(plus1, And(Equal(2), BeNumerically(">", 1))))
+
+		// transform expects custom type
+		type S struct {
+			A int
+			B string
+		}
+		transformer := func(s S) string { return s.B }
+		Expect(S{1, "hi"}).To(WithTransform(transformer, Equal("hi")))
+
+		// transform expects interface
+		errString := func(e error) string { return e.Error() }
+		Expect(errors.New("abc")).To(WithTransform(errString, Equal("abc")))
 	})
 
 	It("works with negative cases", func() {
@@ -25,7 +63,7 @@
 			It("gives a descriptive message", func() {
 				m := WithTransform(plus1, Equal(3))
 				Expect(m.Match(1)).To(BeFalse())
-				Expect(m.FailureMessage(input)).To(Equal("Expected\n    <int>: 2\nto equal\n    <int>: 3"))
+				Expect(m.FailureMessage(1)).To(Equal("Expected\n    <int>: 2\nto equal\n    <int>: 3"))
 			})
 		})
 
@@ -33,7 +71,16 @@
 			It("gives a descriptive message", func() {
 				m := Not(WithTransform(plus1, Equal(3)))
 				Expect(m.Match(2)).To(BeFalse())
-				Expect(m.FailureMessage(input)).To(Equal("Expected\n    <int>: 3\nnot to equal\n    <int>: 3"))
+				Expect(m.FailureMessage(2)).To(Equal("Expected\n    <int>: 3\nnot to equal\n    <int>: 3"))
+			})
+		})
+
+		Context("actual value is incompatible with transform function's argument type", func() {
+			It("gracefully fails if transform cannot be performed", func() {
+				m := WithTransform(plus1, Equal(3))
+				result, err := m.Match("hi") // give it a string but transform expects int; doesn't panic
+				Expect(result).To(BeFalse())
+				Expect(err).To(MatchError("Transform function expects 'int' but we have 'string'"))
 			})
 		})
 	})