# package printer `import "go/printer"` Package printer implements printing of AST nodes.. ## Index * [func Fprint(output io.Writer, fset \*token.FileSet, node interface{}) error](#Fprint) * [type CommentedNode](#CommentedNode) * [type Config](#Config) * [func (cfg \*Config) Fprint(output io.Writer, fset \*token.FileSet, node interface{}) error](#Config.Fprint) * [type Mode](#Mode) ## func Fprint ``` func Fprint(output io.Writer, fset *token.FileSet, node interface{}) error ``` Fprint "pretty-prints" an AST node to output. It calls Config.Fprint with default settings. ``` // Parse source file and extract the AST without comments for // this function, with position information referring to the // file set fset. funcAST, fset := parseFunc("example_test.go", "ExampleFprint") // Print the function body into buffer buf. // The file set is provided to the printer so that it knows // about the original source formatting and can add additional // line breaks where they were present in the source. var buf bytes.Buffer printer.Fprint(&buf, fset, funcAST.Body) // Remove braces {} enclosing the function body, unindent, // and trim leading and trailing white space. s := buf.String() s = s[1 : len(s)-1] s = strings.TrimSpace(strings.Replace(s, "\n\t", "\n", -1)) // Print the cleaned-up body text to stdout. fmt.Println(s) ``` Output: ``` funcAST, fset := parseFunc("example_test.go", "ExampleFprint") var buf bytes.Buffer printer.Fprint(&buf, fset, funcAST.Body) s := buf.String() s = s[1 : len(s)-1] s = strings.TrimSpace(strings.Replace(s, "\n\t", "\n", -1)) fmt.Println(s) ``` ## type CommentedNode ``` type CommentedNode struct { Node interface{} // *ast.File, or ast.Expr, ast.Decl, ast.Spec, or ast.Stmt Comments []*ast.CommentGroup } ``` A CommentedNode bundles an AST node and corresponding comments. It may be provided as argument to any of the Fprint functions. ## type Config ``` type Config struct { Mode Mode // default: 0 Tabwidth int // default: 8 Indent int // default: 0 (all code is indented at least by this much) } ``` A Config node controls the output of Fprint. ## func (\*Config) Fprint ``` func (cfg *Config) Fprint(output io.Writer, fset *token.FileSet, node interface{}) error ``` Fprint "pretty-prints" an AST node to output for a given configuration cfg. Position information is interpreted relative to the file set fset. The node type must be \*ast.File, \*CommentedNode, []ast.Decl, []ast.Stmt, or assignment-compatible to ast.Expr, ast.Decl, ast.Spec, or ast.Stmt. ## type Mode ``` type Mode uint ``` A Mode value is a set of flags (or 0). They control printing. ``` const ( RawFormat Mode = 1 << iota // do not use a tabwriter; if set, UseSpaces is ignored TabIndent // use tabs for indentation independent of UseSpaces UseSpaces // use spaces instead of tabs for alignment SourcePos // emit //line comments to preserve original source positions ) ```