refactors to code generation

internal/gen/gen.go

@@ -1,34 +1,46 @@
 package gen
 
 import (
+	"bytes"
 	"fmt"
-	"io"
+	"go/format"
 
 	"github.com/mmcloughlin/avo/internal/inst"
 )
 
 type Interface interface {
-	Generate(io.Writer, []*inst.Instruction) error
+	Generate([]*inst.Instruction) ([]byte, error)
 }
 
-type Func func(io.Writer, []*inst.Instruction) error
+type Func func([]*inst.Instruction) ([]byte, error)
 
-func (f Func) Generate(w io.Writer, is []*inst.Instruction) error {
-	return f(w, is)
+func (f Func) Generate(is []*inst.Instruction) ([]byte, error) {
+	return f(is)
+}
+
+// GoFmt formats Go code produced from the given generator.
+func GoFmt(i Interface) Interface {
+	return Func(func(is []*inst.Instruction) ([]byte, error) {
+		b, err := i.Generate(is)
+		if err != nil {
+			return nil, err
+		}
+		return format.Source(b)
+	})
 }
 
 type printer struct {
-	w   io.Writer
+	buf bytes.Buffer
 	err error
 }
 
-func (p *printer) printf(format string, args ...interface{}) {
+func (p *printer) Printf(format string, args ...interface{}) {
 	if p.err != nil {
 		return
 	}
-	_, p.err = fmt.Fprintf(p.w, format, args...)
+	_, p.err = fmt.Fprintf(&p.buf, format, args...)
 }
 
-func (p *printer) Err() error {
-	return p.err
+func (p *printer) Result() ([]byte, error) {
+	return p.buf.Bytes(), p.err
 }