如何打印手写AST？

Question

I have a set of XML description of network protocol that I'm trying to generate Go code from, therefore I don't have any existing Go code to work with. All of the examples that make use of go/ast (such as go fmt) take existing code, do some transformation on the AST, then write them back out. Since all I have are XML files, the AST that I'm working with have to be written by hand. The problem is that I cannot get my hand-written AST to output.

Example

package main

import (
    "go/ast"
    "go/printer"
    "go/token"
    "os"
)

func main() {
    f := ast.File{
        Name: ast.NewIdent("foo"),
        Decls: []ast.Decl{
            &ast.GenDecl{
                Tok: token.TYPE,
                Specs: []ast.Spec{
                    &ast.TypeSpec{
                        Name: ast.NewIdent("Bar"),
                        Type: ast.NewIdent("uint32"),
                    },
                },
            },
        },
    }
    fset := token.NewFileSet()
    printer.Fprint(os.Stdout, fset, f)
}

Expected output:

package foo

type Bar uint32

Actual output: nothing

How do I get the AST to print?

Answer 1

Don't ignore errors!

Adding:

err := printer.Fprint(os.Stdout, fset, f)
if err != nil {
    log.Fatal(err)
}

Gives: "go/printer: unsupported node type ast.File" which should have been enough to point you in the right direction.

printer.Fprint's last argument is interface{} so the compiler accepts anything. However, just as parser.ParseFile returns an *ast.File (rather than ast.File) it expects a pointer to a node.

Passing a pointer gives the output you want (note the &ast.File):

package main

import (
    "go/ast"
    "go/printer"
    "go/token"
    "log"
    "os"
)

func main() {
    f := &ast.File{
        Name: ast.NewIdent("foo"),
        Decls: []ast.Decl{
            &ast.GenDecl{
                Tok: token.TYPE,
                Specs: []ast.Spec{
                    &ast.TypeSpec{
                        Name: ast.NewIdent("Bar"),
                        Type: ast.NewIdent("uint32"),
                    },
                },
            },
        },
    }
    fset := token.NewFileSet()
    err := printer.Fprint(os.Stdout, fset, f)
    if err != nil {
        log.Fatal(err)
    }
}

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