Using the go/ast package

I'm attempting to do some metaprogramming in Go and I'm trying to use the go/ast package.

Here's the snippet which is creating the AST:

// Create an AST
fooFn := &ast.FuncDecl{
	Name: &ast.Ident{Name: "foofn"},
	Body: &ast.BlockStmt{
		List: []ast.Stmt{&ast.ExprStmt{
			X: &ast.CallExpr{
				Fun: &ast.Ident{Name: "println"},
				Args: []ast.Expr{
					&ast.BasicLit{Kind: token.INT, Value: "42"},
				},
			},
		}},
	},
}

f := &ast.File{
	Name:  &ast.Ident{Name: "foopackage"},
	Decls: []ast.Decl{fooFn},
	Scope: &ast.Scope{
		Objects: map[string]*ast.Object{
			"Foo": &ast.Object{
				Kind: ast.Fun,
				Name: "Foo",
				Decl: fooFn,
			},
		},
	},
}

See the full code here http://play.golang.org/p/zTJdnqVx__ which should produce this output:

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0xffffffff addr=0x0 pc=0x773ae]

goroutine 16 [running]:
runtime.panic(0x18b5a0, 0x24c8af)
    /tmp/sandbox/go/src/pkg/runtime/panic.c:279 +0x120
go/ast.(*FuncType).Pos(0x0, 0x1c827eb4, 0xfefd02b8, 0x10422360)
    /tmp/sandbox/go/src/pkg/go/ast/ast.go:446 +0x2e
go/ast.(*FuncDecl).Pos(0x10422360, 0x10422360, 0x47, 0xfeee1cb0)
    /tmp/sandbox/go/src/pkg/go/ast/ast.go:933 +0x40
go/printer.(*printer).declList(0x10402840, 0x1040e108, 0x1, 0x1)
    /tmp/sandbox/go/src/pkg/go/printer/nodes.go:1587 +0x180
go/printer.(*printer).file(0x10402840, 0x10442080)
    /tmp/sandbox/go/src/pkg/go/printer/nodes.go:1597 +0x1e0
go/printer.(*printer).printNode(0x10402840, 0x184ba0, 0x10442080, 0x104223c0, 0x0, 0x0)
    /tmp/sandbox/go/src/pkg/go/printer/printer.go:1087 +0xa40
go/printer.(*Config).fprint(0xfeee1e90, 0xfefd02d8, 0x10426180, 0x104181b0, 0x184ba0, 0x10442080, 0x104223c0, 0xfefd02d8, 0x0, 0x0)
    /tmp/sandbox/go/src/pkg/go/printer/printer.go:1226 +0xc0
go/printer.(*Config).Fprint(0xfeee1e90, 0xfefd02d8, 0x10426180, 0x104181b0, 0x184ba0, 0x10442080, 0x0, 0x0)
    /tmp/sandbox/go/src/pkg/go/printer/printer.go:1284 +0xa0
main.main()
    /tmpfs/gosandbox-eb231fa6_4e4e43ec_79a6f993_ce4170ce_07ee185b/prog.go:46 +0x8c0

Edit: here is the working version with FuncDelc.Type set and Scope removed (since I'm only after the source code buffer):

http://play.golang.org/p/B0Xp61rSce

It looks like you may need to provide an ast.FuncType to your ast.FuncDecl. The error appears to be a result of calling IsValid() on the Func property of your (nil) FuncType.