抽象構文木（AST）をいじってフォーマットをかける #golang ref: http://qiita.com/tenntenn/items/8953f2ae80c610b353c8

file0.txt

*ast.BinaryExpr (+)
├── *ast.Ident (x)
└── *ast.Ident (y)

file1.txt

*ast.BinaryExpr (+)
├── *ast.BasicLit (10)
└── *ast.BasicLit (20)

file2.go

type BasicLit struct {
        ValuePos token.Pos   // literal position
        Kind     token.Token // token.INT, token.FLOAT, token.IMAG, token.CHAR, or token.STRING
        Value    string      // literal string; e.g. 42, 0x7f, 3.14, 1e-9, 2.4i, 'a', '\x7f', "foo" or `\m\n\o`
}

file3.go

fset := token.NewFileSet()
expr, err := parser.ParseExprFrom(fset, "sample.go", `x+y`, 0)
if err != nil {
    log.Fatalln("Error:", err)
}

fmt.Println("==== BEFORE ====")
ast.Print(fset, expr)

binaryExpr := expr.(*ast.BinaryExpr)

binaryExpr.X = &ast.BasicLit{
    Kind:  token.INT,
    Value: "10",
}

binaryExpr.Y = &ast.BasicLit{
    Kind:  token.INT,
    Value: "20",
}

fmt.Println("==== AFTER ====")
ast.Print(fset, expr)

file4.txt

==== BEFORE ====
     0  *ast.BinaryExpr {
     1  .  X: *ast.Ident {
     2  .  .  NamePos: sample.go:1:1
     3  .  .  Name: "x"
     4  .  .  Obj: *ast.Object {
     5  .  .  .  Kind: bad
     6  .  .  .  Name: ""
     7  .  .  }
     8  .  }
     9  .  OpPos: sample.go:1:2
    10  .  Op: +
    11  .  Y: *ast.Ident {
    12  .  .  NamePos: sample.go:1:3
    13  .  .  Name: "y"
    14  .  .  Obj: *(obj @ 4)
    15  .  }
    16  }
==== AFTER ====
     0  *ast.BinaryExpr {
     1  .  X: *ast.BasicLit {
     2  .  .  ValuePos: -
     3  .  .  Kind: INT
     4  .  .  Value: "10"
     5  .  }
     6  .  OpPos: sample.go:1:2
     7  .  Op: +
     8  .  Y: *ast.BasicLit {
     9  .  .  ValuePos: -
    10  .  .  Kind: INT
    11  .  .  Value: "20"
    12  .  }
    13  }

file5.go

func Node(dst io.Writer, fset *token.FileSet, node interface{}) error
func Source(src []byte) ([]byte, error)

file6.go

if err := format.Node(os.Stdout, fset, expr); err != nil {
    log.Fatalln("Error:", err)
}

file7.txt

10 + 20

file8.go

package main

import (
    "fmt"
    "go/ast"
    "go/format"
    "go/parser"
    "go/token"
    "log"
    "os"
)

func main() {
    fset := token.NewFileSet()
    expr, err := parser.ParseExprFrom(fset, "sample.go", `x+y`, 0)
    if err != nil {
        log.Fatalln("Error:", err)
    }

    fmt.Println("==== BEFORE ====")
    ast.Print(fset, expr)

    binaryExpr := expr.(*ast.BinaryExpr)

    binaryExpr.X = &ast.BasicLit{
        Kind:  token.INT,
        Value: "10",
    }

    binaryExpr.Y = &ast.BasicLit{
        Kind:  token.INT,
        Value: "20",
    }

    fmt.Println("==== AFTER ====")
    ast.Print(fset, expr)

    if err := format.Node(os.Stdout, fset, expr); err != nil {
        log.Fatalln("Error:", err)
    }
}