src/go/types/exprstring.go

// Copyright 2013 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 file.

// This file implements printing of expressions.

package types

import (
	"bytes"
	"go/ast"
)

// ExprString returns the (possibly shortened) string representation for x.
// Shortened representations are suitable for user interfaces but may not
// necessarily follow Go syntax.
func ExprString(x ast.Expr) string {
	var buf bytes.Buffer
	WriteExpr(&buf, x)
	return buf.String()
}

// WriteExpr writes the (possibly shortened) string representation for x to buf.
// Shortened representations are suitable for user interfaces but may not
// necessarily follow Go syntax.
func WriteExpr(buf *bytes.Buffer, x ast.Expr) {
	// The AST preserves source-level parentheses so there is
	// no need to introduce them here to correct for different
	// operator precedences. (This assumes that the AST was
	// generated by a Go parser.)

	switch x := x.(type) {
	default:
		buf.WriteString("(bad expr)") // nil, ast.BadExpr, ast.KeyValueExpr

	case *ast.Ident:
		buf.WriteString(x.Name)

	case *ast.Ellipsis:
		buf.WriteString("...")
		if x.Elt != nil {
			WriteExpr(buf, x.Elt)
		}

	case *ast.BasicLit:
		buf.WriteString(x.Value)

	case *ast.FuncLit:
		buf.WriteByte('(')
		WriteExpr(buf, x.Type)
		buf.WriteString(" literal)") // shortened

	case *ast.CompositeLit:
		buf.WriteByte('(')
		WriteExpr(buf, x.Type)
		buf.WriteString(" literal)") // shortened

	case *ast.ParenExpr:
		buf.WriteByte('(')
		WriteExpr(buf, x.X)
		buf.WriteByte(')')

	case *ast.SelectorExpr:
		WriteExpr(buf, x.X)
		buf.WriteByte('.')
		buf.WriteString(x.Sel.Name)

	case *ast.IndexExpr:
		WriteExpr(buf, x.X)
		buf.WriteByte('[')
		WriteExpr(buf, x.Index)
		buf.WriteByte(']')

	case *ast.SliceExpr:
		WriteExpr(buf, x.X)
		buf.WriteByte('[')
		if x.Low != nil {
			WriteExpr(buf, x.Low)
		}
		buf.WriteByte(':')
		if x.High != nil {
			WriteExpr(buf, x.High)
		}
		if x.Slice3 {
			buf.WriteByte(':')
			if x.Max != nil {
				WriteExpr(buf, x.Max)
			}
		}
		buf.WriteByte(']')

	case *ast.TypeAssertExpr:
		WriteExpr(buf, x.X)
		buf.WriteString(".(")
		WriteExpr(buf, x.Type)
		buf.WriteByte(')')

	case *ast.CallExpr:
		WriteExpr(buf, x.Fun)
		buf.WriteByte('(')
		for i, arg := range x.Args {
			if i > 0 {
				buf.WriteString(", ")
			}
			WriteExpr(buf, arg)
		}
		if x.Ellipsis.IsValid() {
			buf.WriteString("...")
		}
		buf.WriteByte(')')

	case *ast.StarExpr:
		buf.WriteByte('*')
		WriteExpr(buf, x.X)

	case *ast.UnaryExpr:
		buf.WriteString(x.Op.String())
		WriteExpr(buf, x.X)

	case *ast.BinaryExpr:
		WriteExpr(buf, x.X)
		buf.WriteByte(' ')
		buf.WriteString(x.Op.String())
		buf.WriteByte(' ')
		WriteExpr(buf, x.Y)

	case *ast.ArrayType:
		buf.WriteByte('[')
		if x.Len != nil {
			WriteExpr(buf, x.Len)
		}
		buf.WriteByte(']')
		WriteExpr(buf, x.Elt)

	case *ast.StructType:
		buf.WriteString("struct{")
		writeFieldList(buf, x.Fields, "; ", false)
		buf.WriteByte('}')

	case *ast.FuncType:
		buf.WriteString("func")
		writeSigExpr(buf, x)

	case *ast.InterfaceType:
		buf.WriteString("interface{")
		writeFieldList(buf, x.Methods, "; ", true)
		buf.WriteByte('}')

	case *ast.MapType:
		buf.WriteString("map[")
		WriteExpr(buf, x.Key)
		buf.WriteByte(']')
		WriteExpr(buf, x.Value)

	case *ast.ChanType:
		var s string
		switch x.Dir {
		case ast.SEND:
			s = "chan<- "
		case ast.RECV:
			s = "<-chan "
		default:
			s = "chan "
		}
		buf.WriteString(s)
		WriteExpr(buf, x.Value)
	}
}

func writeSigExpr(buf *bytes.Buffer, sig *ast.FuncType) {
	buf.WriteByte('(')
	writeFieldList(buf, sig.Params, ", ", false)
	buf.WriteByte(')')

	res := sig.Results
	n := res.NumFields()
	if n == 0 {
		// no result
		return
	}

	buf.WriteByte(' ')
	if n == 1 && len(res.List[0].Names) == 0 {
		// single unnamed result
		WriteExpr(buf, res.List[0].Type)
		return
	}

	// multiple or named result(s)
	buf.WriteByte('(')
	writeFieldList(buf, res, ", ", false)
	buf.WriteByte(')')
}

func writeFieldList(buf *bytes.Buffer, fields *ast.FieldList, sep string, iface bool) {
	for i, f := range fields.List {
		if i > 0 {
			buf.WriteString(sep)
		}

		// field list names
		for i, name := range f.Names {
			if i > 0 {
				buf.WriteString(", ")
			}
			buf.WriteString(name.Name)
		}

		// types of interface methods consist of signatures only
		if sig, _ := f.Type.(*ast.FuncType); sig != nil && iface {
			writeSigExpr(buf, sig)
			continue
		}

		// named fields are separated with a blank from the field type
		if len(f.Names) > 0 {
			buf.WriteByte(' ')
		}

		WriteExpr(buf, f.Type)

		// ignore tag
	}
}