Add support for filters.

CHANGELOG.md

@@ -9,6 +9,17 @@ The format is based on [Keep a Changelog], and this project adheres to
 [Keep a Changelog]: https://keepachangelog.com/en/1.0.0/
 [Semantic Versioning]: https://semver.org/spec/v2.0.0.html
 
+## [Unreleased]
+
+### Added
+
+- Added support for filters, allowing custom rendering on a per-value basis
+- Added a built-in filter for `reflect.Type`
+
+### Fixed
+
+- Fixed a bug whereby IO errors were not propagated to the caller
+
 ## [0.2.0] - 2019-01-19
 
 ### Added

README.md

@@ -6,9 +6,17 @@
 [![GoDoc](https://godoc.org/github.com/dogmatiq/dapper?status.svg)](https://godoc.org/github.com/dogmatiq/dapper)
 [![Go Report Card](https://goreportcard.com/badge/github.com/dogmatiq/dapper)](https://goreportcard.com/report/github.com/dogmatiq/dapper)
 
-Dapper is a pretty-printer for Go values that aims to produce the shortest
-possible output without ambiguity. Additionally, the output is deterministic,
-which allows for the generation of human-readable diffs using standard tools.
+Dapper is a pretty-printer for Go values.
+
+It is not intended to be used directly as a debugging tool, but as a library
+for applications that need to describe Go values to humans, such as testing
+frameworks.
+
+Some features include:
+
+- Concise formatting, without type ambiguity
+- Deterministic output, useful for generating diffs using standard tools
+- A filtering system for producing customized output on a per-value basis
 
 ## Example
 

array.go

@@ -8,8 +8,8 @@ import (
 )
 
 // visitArray formats values with a kind of reflect.Array or Slice.
-func (vis *visitor) visitArray(w io.Writer, v value) {
-	if v.IsAmbiguousType {
+func (vis *visitor) visitArray(w io.Writer, v Value) {
+	if v.IsAmbiguousType() {
 		vis.write(w, v.TypeName())
 	}
 
@@ -23,11 +23,31 @@ func (vis *visitor) visitArray(w io.Writer, v value) {
 	vis.write(w, "}")
 }
 
-func (vis *visitor) visitArrayValues(w io.Writer, v value) {
-	ambiguous := v.Type.Elem().Kind() == reflect.Interface
+func (vis *visitor) visitArrayValues(w io.Writer, v Value) {
+	staticType := v.DynamicType.Elem()
+	isInterface := staticType.Kind() == reflect.Interface
 
 	for i := 0; i < v.Value.Len(); i++ {
-		vis.visit(w, v.Value.Index(i), ambiguous)
+		elem := v.Value.Index(i)
+
+		// unwrap interface values so that elem has it's actual type/kind, and not
+		// that of reflect.Interface.
+		if isInterface && !elem.IsNil() {
+			elem = elem.Elem()
+		}
+
+		vis.visit(
+			w,
+			Value{
+				Value:                  elem,
+				DynamicType:            elem.Type(),
+				StaticType:             staticType,
+				IsAmbiguousDynamicType: isInterface,
+				IsAmbiguousStaticType:  false,
+				IsUnexported:           v.IsUnexported,
+			},
+		)
+
 		vis.write(w, "\n")
 	}
 }

filter.go

@@ -0,0 +1,82 @@
+package dapper
+
+import (
+	"io"
+	"reflect"
+
+	"github.com/dogmatiq/iago"
+)
+
+// Filter is a function that provides custom formatting logic for specific
+// values.
+//
+// It writes a formatted representation of v to w, and returns the number of
+// bytes written.
+//
+// If the number of bytes written is non-zero, the default formatting logic is
+// skipped.
+//
+// Particular attention should be paid to the v.IsUnexported field. If this flag
+// is true, many operations on v.Value are unavailable.
+type Filter func(w io.Writer, v Value) (int, error)
+
+// reflectTypeType is the reflect.Type for reflect.Type itself.
+var reflectTypeType = reflect.TypeOf((*reflect.Type)(nil)).Elem()
+
+// ReflectTypeFilter is a filter that formats reflect.Type values.
+func ReflectTypeFilter(w io.Writer, v Value) (n int, err error) {
+	defer iago.Recover(&err)
+
+	if !v.DynamicType.Implements(reflectTypeType) {
+		return 0, nil
+	}
+
+	if v.DynamicType.Kind() == reflect.Interface && v.Value.IsNil() {
+		return 0, nil
+	}
+
+	ambiguous := false
+
+	if v.IsAmbiguousStaticType {
+		// always render the type if the static type is ambiguous
+		ambiguous = true
+	} else if v.IsAmbiguousDynamicType {
+		// only consider the dynamic type to be ambiguous if the static type isn't reflect.Type
+		// we're not really concerned with rendering the underlying implementation's type.
+		ambiguous = v.StaticType != reflectTypeType
+	}
+
+	if ambiguous {
+		n += iago.MustWriteString(w, "reflect.Type(")
+	}
+
+	if v.IsUnexported {
+		n += iago.MustWriteString(w, "<unknown>")
+	} else {
+		t := v.Value.Interface().(reflect.Type)
+
+		if s := t.PkgPath(); s != "" {
+			n += iago.MustWriteString(w, s)
+			n += iago.MustWriteString(w, ".")
+		}
+
+		if s := t.Name(); s != "" {
+			n += iago.MustWriteString(w, s)
+		} else {
+			n += iago.MustWriteString(w, t.String())
+		}
+
+	}
+
+	// always render the pointer value for the type, this way when the field is
+	// unexported we still get something we can compare to known types instead of a
+	// rendering of the reflect.rtype struct.
+	n += iago.MustWriteString(w, " ")
+	n += iago.MustWriteString(w, formatPointerHex(v.Value.Pointer(), false))
+
+	if ambiguous {
+		n += iago.MustWriteString(w, ")")
+	}
+
+	return
+}

filter_test.go

@@ -0,0 +1,101 @@
+package dapper_test
+
+import (
+	"fmt"
+	"reflect"
+	"testing"
+)
+
+type reflectType struct {
+	Exported   reflect.Type
+	unexported reflect.Type
+}
+
+var (
+	intType          = reflect.TypeOf(0)
+	intTypePointer   = formatReflectTypePointer(intType)
+	mapType          = reflect.TypeOf(map[string]string{})
+	mapTypePointer   = formatReflectTypePointer(mapType)
+	namedType        = reflect.TypeOf(named{})
+	namedTypePointer = formatReflectTypePointer(namedType)
+)
+
+func formatReflectTypePointer(t reflect.Type) string {
+	return fmt.Sprintf("0x%x", reflect.ValueOf(t).Pointer())
+}
+
+func TestPrinter_ReflectTypeFilter(t *testing.T) {
+	test(
+		t,
+		"built-in type",
+		intType,
+		"reflect.Type(int "+intTypePointer+")",
+	)
+
+	test(
+		t,
+		"built-in parameterized type",
+		mapType,
+		"reflect.Type(map[string]string "+mapTypePointer+")",
+	)
+
+	test(
+		t,
+		"named type",
+		reflect.TypeOf(named{}),
+		"reflect.Type(github.com/dogmatiq/dapper_test.named "+namedTypePointer+")",
+	)
+
+	typ := reflect.TypeOf(struct{ Int int }{})
+	test(
+		t,
+		"anonymous struct",
+		typ,
+		"reflect.Type(struct { Int int } "+formatReflectTypePointer(typ)+")",
+	)
+
+	typ = reflect.TypeOf((*interface{ Int() int })(nil)).Elem()
+	test(
+		t,
+		"anonymous interface",
+		typ,
+		"reflect.Type(interface { Int() int } "+formatReflectTypePointer(typ)+")",
+	)
+
+	test(
+		t,
+		"includes type when in an anonymous struct",
+		struct {
+			Type reflect.Type
+		}{
+			reflect.TypeOf(0),
+		},
+		"{",
+		"    Type: reflect.Type(int "+intTypePointer+")",
+		"}",
+	)
+
+	test(
+		t,
+		"does not include type if static type is also reflect.Type",
+		reflectType{
+			Exported: reflect.TypeOf(0),
+		},
+		"dapper_test.reflectType{",
+		"    Exported:   int "+intTypePointer,
+		"    unexported: nil",
+		"}",
+	)
+
+	test(
+		t,
+		"still renders the pointer address when the value is unexported",
+		reflectType{
+			unexported: reflect.TypeOf(0),
+		},
+		"dapper_test.reflectType{",
+		"    Exported:   nil",
+		"    unexported: <unknown> "+intTypePointer,
+		"}",
+	)
+}

interface.go

@@ -5,17 +5,20 @@ import (
 )
 
 // visitInterface formats values with a kind of reflect.Interface.
-func (vis *visitor) visitInterface(w io.Writer, v value) {
-	if v.Value.IsNil() {
-		if v.IsAmbiguousType {
-			vis.write(w, v.TypeName())
-			vis.write(w, "(nil)")
-		} else {
-			vis.write(w, "nil")
-		}
-
-		return
+func (vis *visitor) visitInterface(w io.Writer, v Value) {
+	if !v.Value.IsNil() {
+		// this should never happen, a more appropraite visit method should have been
+		// chosen based on the value's dynamic type.
+		panic("unexpectedly called visitInterface() with non-nil interface")
 	}
 
-	vis.visit(w, v.Value.Elem(), v.IsAmbiguousType)
+	// for a nil interface, we only want to render the type if the STATIC type is
+	// ambigious, since the only information we have available is the interface
+	// type itself, not the actual implementation's type.
+	if v.IsAmbiguousStaticType {
+		vis.write(w, v.TypeName())
+		vis.write(w, "(nil)")
+	} else {
+		vis.write(w, "nil")
+	}
 }

interface_test.go

@@ -6,10 +6,13 @@ type interfaces struct {
 	Iface interface{}
 }
 
+type iface interface{}
+
 func TestPrinter_Interface(t *testing.T) {
 	// note that capturing a reflect.Value of a nil interface does NOT produces a
 	// value with a "kind" of reflect.Invalid, NOT reflect.Interface.
 	test(t, "nil interface", interface{}(nil), "interface{}(nil)")
+	test(t, "nil named interface", iface(nil), "interface{}(nil)") // interface information is shed when passed to Printer.Write().
 
 	test(
 		t,