Add cmp/internal/function package (#35)

Unify the common logic for identifying the type of function.
This logic is helpful since Go lacks generics, so it is hard
to query whether a function is of the signature: func(T, T) bool

1 files changed, 49 insertions, 0 deletions

diff --git a/cmp/internal/function/func.go b/cmp/internal/function/func.go
new file mode 100644
index 0000000..4c35ff1
--- /dev/null
+++ b/cmp/internal/function/func.go
@@ -0,0 +1,49 @@
+// Copyright 2017, The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE.md file.
+
+// Package function identifies function types.
+package function
+
+import "reflect"
+
+type funcType int
+
+const (
+	_ funcType = iota
+
+	ttbFunc // func(T, T) bool
+	tibFunc // func(T, I) bool
+	trFunc  // func(T) R
+
+	Equal           = ttbFunc // func(T, T) bool
+	EqualAssignable = tibFunc // func(T, I) bool; encapsulates func(T, T) bool
+	Transformer     = trFunc  // func(T) R
+	ValueFilter     = ttbFunc // func(T, T) bool
+	Less            = ttbFunc // func(T, T) bool
+)
+
+var boolType = reflect.TypeOf(true)
+
+// IsType reports whether the reflect.Type is of the specified function type.
+func IsType(t reflect.Type, ft funcType) bool {
+	if t == nil || t.Kind() != reflect.Func || t.IsVariadic() {
+		return false
+	}
+	ni, no := t.NumIn(), t.NumOut()
+	switch ft {
+	case ttbFunc: // func(T, T) bool
+		if ni == 2 && no == 1 && t.In(0) == t.In(1) && t.Out(0) == boolType {
+			return true
+		}
+	case tibFunc: // func(T, I) bool
+		if ni == 2 && no == 1 && t.In(0).AssignableTo(t.In(1)) && t.Out(0) == boolType {
+			return true
+		}
+	case trFunc: // func(T) R
+		if ni == 1 && no == 1 {
+			return true
+		}
+	}
+	return false
+}