Awesome

Jennifer

Jennifer is a code generator for Go.

package main

import (
    "fmt"

    . "github.com/dave/jennifer/jen"
)

func main() {
	f := NewFile("main")
	f.Func().Id("main").Params().Block(
		Qual("fmt", "Println").Call(Lit("Hello, world")),
	)
	fmt.Printf("%#v", f)
}

Output:

package main

import "fmt"

func main() {
	fmt.Println("Hello, world")
}

Install

go get -u github.com/dave/jennifer/jen

Need help?

If you get stuck, have a question, would like a code review, or just want a chat: I'm happy to help! Feel free to open an issue, email me or mention @dave in your PR.

Examples

Jennifer has a comprehensive suite of examples - see godoc for an index. Here's some examples of jennifer being used in the real-world:

• genjen (which generates much of jennifer, using data in data.go)
• zerogen
• go-contentful-generator

Rendering

For testing, a File or Statement can be rendered with the fmt package using the %#v verb.

c := Id("a").Call(Lit("b"))
fmt.Printf("%#v", c)
// Output:
// a("b")

This is not recommended for use in production because any error will cause a panic. For production use, File.Render or File.Save are preferred.

Identifiers

Identifiers Keywords Operators Braces Parentheses Control flow Collections Literals Comments Generics Helpers Misc File

Id

Id renders an identifier.

c := If(Id("i").Op("==").Id("j")).Block(
	Return(Id("i")),
)
fmt.Printf("%#v", c)
// Output:
// if i == j {
// 	return i
// }

Dot

Dot renders a period followed by an identifier. Use for fields and selectors.

c := Qual("a.b/c", "Foo").Call().Dot("Bar").Index(Lit(0)).Dot("Baz")
fmt.Printf("%#v", c)
// Output:
// c.Foo().Bar[0].Baz

Qual

Qual renders a qualified identifier.

c := Qual("encoding/gob", "NewEncoder").Call()
fmt.Printf("%#v", c)
// Output:
// gob.NewEncoder()

Imports are automatically added when used with a File. If the path matches the local path, the package name is omitted. If package names conflict they are automatically renamed.

f := NewFilePath("a.b/c")
f.Func().Id("init").Params().Block(
	Qual("a.b/c", "Foo").Call().Comment("Local package - name is omitted."),
	Qual("d.e/f", "Bar").Call().Comment("Import is automatically added."),
	Qual("g.h/f", "Baz").Call().Comment("Colliding package name is renamed."),
)
fmt.Printf("%#v", f)
// Output:
// package c
//
// import (
// 	f "d.e/f"
// 	f1 "g.h/f"
// )
//
// func init() {
// 	Foo()    // Local package - name is omitted.
// 	f.Bar()  // Import is automatically added.
// 	f1.Baz() // Colliding package name is renamed.
// }

Note that it is not possible to reliably determine the package name given an arbitrary package path, so a sensible name is guessed from the path and added as an alias. The names of all standard library packages are known so these do not need to be aliased. If more control is needed of the aliases, see File.ImportName or File.ImportAlias.

List

List renders a comma separated list. Use for multiple return functions.

c := List(Id("a"), Err()).Op(":=").Id("b").Call()
fmt.Printf("%#v", c)
// Output:
// a, err := b()

Keywords

Identifiers Keywords Operators Braces Parentheses Control flow Collections Literals Comments Generics Helpers Misc File

Simple keywords, predeclared identifiers and built-in functions are self explanatory:

Built-in functions take a list of parameters and render them appropriately:

c := Id("a").Op("=").Append(Id("a"), Id("b").Op("..."))
fmt.Printf("%#v", c)
// Output:
// a = append(a, b...)

Special cases for If, For, Interface, Struct, Switch, Case, Return and Map are explained below.

Operators

Identifiers Keywords Operators Braces Parentheses Control flow Collections Literals Comments Generics Helpers Misc File

Op renders the provided operator / token.

c := Id("a").Op(":=").Id("b").Call()
fmt.Printf("%#v", c)
// Output:
// a := b()

c := Id("a").Op("=").Op("*").Id("b")
fmt.Printf("%#v", c)
// Output:
// a = *b

c := Id("a").Call(Id("b").Op("..."))
fmt.Printf("%#v", c)
// Output:
// a(b...)

c := If(Parens(Id("a").Op("||").Id("b")).Op("&&").Id("c")).Block()
fmt.Printf("%#v", c)
// Output:
// if (a || b) && c {
// }

Braces

Identifiers Keywords Operators Braces Parentheses Control flow Collections Literals Comments Generics Helpers Misc File

Several methods render curly braces, summarized below:

Block

Block renders a statement list enclosed by curly braces. Use for code blocks.

c := Func().Id("foo").Params().String().Block(
	Id("a").Op("=").Id("b"),
	Id("b").Op("++"),
	Return(Id("b")),
)
fmt.Printf("%#v", c)
// Output:
// func foo() string {
// 	a = b
// 	b++
// 	return b
// }

c := If(Id("a").Op(">").Lit(10)).Block(
	Id("a").Op("=").Id("a").Op("/").Lit(2),
)
fmt.Printf("%#v", c)
// Output:
// if a > 10 {
// 	a = a / 2
// }

A special case applies when used directly after Case or Default, where the braces are omitted. This allows use in switch and select statements. See example.

Interface, Struct

Interface and Struct render the keyword followed by a statement list enclosed by curly braces.

c := Var().Id("a").Interface()
fmt.Printf("%#v", c)
// Output:
// var a interface{}

c := Type().Id("a").Interface(
	Id("b").Params().String(),
)
fmt.Printf("%#v", c)
// Output:
// type a interface {
// 	b() string
// }

c := Id("c").Op(":=").Make(Chan().Struct())
fmt.Printf("%#v", c)
// Output:
// c := make(chan struct{})

c := Type().Id("foo").Struct(
	List(Id("x"), Id("y")).Int(),
	Id("u").Float32(),
)
fmt.Printf("%#v", c)
// Output:
// type foo struct {
// 	x, y int
// 	u    float32
// }

Parentheses

Identifiers Keywords Operators Braces Parentheses Control flow Collections Literals Comments Generics Helpers Misc File

Several methods output parenthesis, summarized below:

Call

Call renders a comma separated list enclosed by parenthesis. Use for function calls.

c := Qual("fmt", "Printf").Call(
	Lit("%#v: %T\n"),
	Id("a"),
	Id("b"),
)
fmt.Printf("%#v", c)
// Output:
// fmt.Printf("%#v: %T\n", a, b)

Params

Params renders a comma separated list enclosed by parenthesis. Use for function parameters and method receivers.

c := Func().Params(
	Id("a").Id("A"),
).Id("foo").Params(
	Id("b"),
	Id("c").String(),
).String().Block(
	Return(Id("b").Op("+").Id("c")),
)
fmt.Printf("%#v", c)
// Output:
// func (a A) foo(b, c string) string {
// 	return b + c
// }

Defs

Defs renders a statement list enclosed in parenthesis. Use for definition lists.

c := Const().Defs(
	Id("a").Op("=").Lit("a"),
	Id("b").Op("=").Lit("b"),
)
fmt.Printf("%#v", c)
// Output:
// const (
// 	a = "a"
// 	b = "b"
// )

Parens

Parens renders a single item in parenthesis. Use for type conversion or to specify evaluation order.

c := Id("b").Op(":=").Index().Byte().Parens(Id("s"))
fmt.Printf("%#v", c)
// Output:
// b := []byte(s)

c := Id("a").Op("/").Parens(Id("b").Op("+").Id("c"))
fmt.Printf("%#v", c)
// Output:
// a / (b + c)

Assert

Assert renders a period followed by a single item enclosed by parenthesis. Use for type assertions.

c := List(Id("b"), Id("ok")).Op(":=").Id("a").Assert(Bool())
fmt.Printf("%#v", c)
// Output:
// b, ok := a.(bool)

Control flow

Identifiers Keywords Operators Braces Parentheses Control flow Collections Literals Comments Generics Helpers Misc File

If, For

If and For render the keyword followed by a semicolon separated list.

c := If(
	Err().Op(":=").Id("a").Call(),
	Err().Op("!=").Nil(),
).Block(
	Return(Err()),
)
fmt.Printf("%#v", c)
// Output:
// if err := a(); err != nil {
// 	return err
// }

c := For(
	Id("i").Op(":=").Lit(0),
	Id("i").Op("<").Lit(10),
	Id("i").Op("++"),
).Block(
	Qual("fmt", "Println").Call(Id("i")),
)
fmt.Printf("%#v", c)
// Output:
// for i := 0; i < 10; i++ {
// 	fmt.Println(i)
// }

Switch, Select

Switch, Select, Case and Block are used to build switch or select statements:

c := Switch(Id("value").Dot("Kind").Call()).Block(
	Case(Qual("reflect", "Float32"), Qual("reflect", "Float64")).Block(
		Return(Lit("float")),
	),
	Case(Qual("reflect", "Bool")).Block(
		Return(Lit("bool")),
	),
	Case(Qual("reflect", "Uintptr")).Block(
		Fallthrough(),
	),
	Default().Block(
		Return(Lit("none")),
	),
)
fmt.Printf("%#v", c)
// Output:
// switch value.Kind() {
// case reflect.Float32, reflect.Float64:
// 	return "float"
// case reflect.Bool:
// 	return "bool"
// case reflect.Uintptr:
// 	fallthrough
// default:
// 	return "none"
// }

Return

Return renders the keyword followed by a comma separated list.

c := Return(Id("a"), Id("b"))
fmt.Printf("%#v", c)
// Output:
// return a, b

Collections

Identifiers Keywords Operators Braces Parentheses Control flow Collections Literals Comments Generics Helpers Misc File

Map

Map renders the keyword followed by a single item enclosed by square brackets. Use for map definitions.

c := Id("a").Op(":=").Map(String()).String().Values()
fmt.Printf("%#v", c)
// Output:
// a := map[string]string{}

Index

Index renders a colon separated list enclosed by square brackets. Use for array / slice indexes and definitions.

c := Var().Id("a").Index().String()
fmt.Printf("%#v", c)
// Output:
// var a []string

c := Id("a").Op(":=").Id("b").Index(Lit(0), Lit(1))
fmt.Printf("%#v", c)
// Output:
// a := b[0:1]

c := Id("a").Op(":=").Id("b").Index(Lit(1), Empty())
fmt.Printf("%#v", c)
// Output:
// a := b[1:]

Values

Values renders a comma separated list enclosed by curly braces. Use for slice or composite literals.

c := Index().String().Values(Lit("a"), Lit("b"))
fmt.Printf("%#v", c)
// Output:
// []string{"a", "b"}

Dict renders as key/value pairs. Use with Values for map or composite literals.

c := Map(String()).String().Values(Dict{
	Lit("a"):	Lit("b"),
	Lit("c"):	Lit("d"),
})
fmt.Printf("%#v", c)
// Output:
// map[string]string{
// 	"a": "b",
// 	"c": "d",
// }

c := Op("&").Id("Person").Values(Dict{
	Id("Age"):	Lit(1),
	Id("Name"):	Lit("a"),
})
fmt.Printf("%#v", c)
// Output:
// &Person{
// 	Age:  1,
// 	Name: "a",
// }

DictFunc executes a func(Dict) to generate the value.

c := Id("a").Op(":=").Map(String()).String().Values(DictFunc(func(d Dict) {
	d[Lit("a")] = Lit("b")
	d[Lit("c")] = Lit("d")
}))
fmt.Printf("%#v", c)
// Output:
// a := map[string]string{
// 	"a": "b",
// 	"c": "d",
// }

Note: the items are ordered by key when rendered to ensure repeatable code.

Literals

Identifiers Keywords Operators Braces Parentheses Control flow Collections Literals Comments Generics Helpers Misc File

Lit

Lit renders a literal. Lit supports only built-in types (bool, string, int, complex128, float64, float32, int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64, uintptr and complex64). Passing any other type will panic.

c := Id("a").Op(":=").Lit("a")
fmt.Printf("%#v", c)
// Output:
// a := "a"

c := Id("a").Op(":=").Lit(1.5)
fmt.Printf("%#v", c)
// Output:
// a := 1.5

LitFunc generates the value to render by executing the provided function.

c := Id("a").Op(":=").LitFunc(func() interface{} { return 1 + 1 })
fmt.Printf("%#v", c)
// Output:
// a := 2

For the default constant types (bool, int, float64, string, complex128), Lit will render the untyped constant.

For all other built-in types (float32, int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64, uintptr, complex64), Lit will also render the type.

The built-in alias types byte and rune need a special case. LitRune and LitByte render rune and byte literals.

Comments

Identifiers Keywords Operators Braces Parentheses Control flow Collections Literals Comments Generics Helpers Misc File

Comment

Comment adds a comment. If the provided string contains a newline, the comment is formatted in multiline style.

f := NewFile("a")
f.Comment("Foo returns the string \"foo\"")
f.Func().Id("Foo").Params().String().Block(
	Return(Lit("foo")).Comment("return the string foo"),
)
fmt.Printf("%#v", f)
// Output:
// package a
//
// // Foo returns the string "foo"
// func Foo() string {
// 	return "foo" // return the string foo
// }

c := Comment("a\nb")
fmt.Printf("%#v", c)
// Output:
// /*
// a
// b
// */

If the comment string starts with "//" or "/*", the automatic formatting is disabled and the string is rendered directly.

c := Id("foo").Call(Comment("/* inline */")).Comment("//no-space")
fmt.Printf("%#v", c)
// Output:
// foo( /* inline */ ) //no-space

Commentf

Commentf adds a comment, using a format string and a list of parameters.

name := "foo"
val := "bar"
c := Id(name).Op(":=").Lit(val).Commentf("%s is the string \"%s\"", name, val)
fmt.Printf("%#v", c)
// Output:
// foo := "bar" // foo is the string "bar"

Generics

Identifiers Keywords Operators Braces Parentheses Control flow Collections Literals Comments Generics Helpers Misc File

It is hoped that with the introduction of generics with Go 1.18, the need to generate code will be reduced. However, for the sake of completeness, we now support generics including the any and comparable predeclared identifiers, and the Types and Union lists. To emit the approximation (~) token, use Op("~").

Types

Types renders a comma separated list enclosed by square brackets. Use for type parameters and constraints.

Union

Union renders a pipe separated list. Use for union type constraints.

Examples

c := Func().Id("Keys").Types(
	Id("K").Comparable(),
	Id("V").Any(),
).Params(
	Id("m").Map(Id("K")).Id("V"),
).Index().Id("K").Block()
fmt.Printf("%#v", c)
// Output:
// func Keys[K comparable, V any](m map[K]V) []K {}

c := Return(Id("Keys").Types(Int(), String()).Call(Id("m")))
fmt.Printf("%#v", c)
// Output:
// return Keys[int, string](m)

c := Type().Id("PredeclaredSignedInteger").Interface(
	Union(Int(), Int8(), Int16(), Int32(), Int64()),
)
fmt.Printf("%#v", c)
// Output:
// type PredeclaredSignedInteger interface {
//	int | int8 | int16 | int32 | int64
// }

c := Type().Id("AnyString").Interface(
	Op("~").String(),
)
fmt.Printf("%#v", c)
// Output:
// type AnyString interface {
//	~string
// }

Helpers

Identifiers Keywords Operators Braces Parentheses Control flow Collections Literals Comments Generics Helpers Misc File

Func methods

All constructs that accept a variadic list of items are paired with GroupFunc functions that accept a func(*Group). Use for embedding logic.

c := Id("numbers").Op(":=").Index().Int().ValuesFunc(func(g *Group) {
	for i := 0; i <= 5; i++ {
		g.Lit(i)
	}
})
fmt.Printf("%#v", c)
// Output:
// numbers := []int{0, 1, 2, 3, 4, 5}

increment := true
name := "a"
c := Func().Id("a").Params().BlockFunc(func(g *Group) {
	g.Id(name).Op("=").Lit(1)
	if increment {
		g.Id(name).Op("++")
	} else {
		g.Id(name).Op("--")
	}
})
fmt.Printf("%#v", c)
// Output:
// func a() {
// 	a = 1
// 	a++
// }

Add

Add appends the provided items to the statement.

ptr := Op("*")
c := Id("a").Op("=").Add(ptr).Id("b")
fmt.Printf("%#v", c)
// Output:
// a = *b

a := Id("a")
i := Int()
c := Var().Add(a, i)
fmt.Printf("%#v", c)
// Output:
// var a int

Do

Do calls the provided function with the statement as a parameter. Use for embedding logic.

f := func(name string, isMap bool) *Statement {
	return Id(name).Op(":=").Do(func(s *Statement) {
		if isMap {
			s.Map(String()).String()
		} else {
			s.Index().String()
		}
	}).Values()
}
fmt.Printf("%#v\n%#v", f("a", true), f("b", false))
// Output:
// a := map[string]string{}
// b := []string{}

Misc

Identifiers Keywords Operators Braces Parentheses Control flow Collections Literals Comments Generics Helpers Misc File

Tag

Tag renders a struct tag

c := Type().Id("foo").Struct(
	Id("A").String().Tag(map[string]string{"json": "a"}),
	Id("B").Int().Tag(map[string]string{"json": "b", "bar": "baz"}),
)
fmt.Printf("%#v", c)
// Output:
// type foo struct {
// 	A string `json:"a"`
// 	B int    `bar:"baz" json:"b"`
// }

Note: the items are ordered by key when rendered to ensure repeatable code.

Null

Null adds a null item. Null items render nothing and are not followed by a separator in lists.

In lists, nil will produce the same effect.

c := Func().Id("foo").Params(
	nil,
	Id("s").String(),
	Null(),
	Id("i").Int(),
).Block()
fmt.Printf("%#v", c)
// Output:
// func foo(s string, i int) {}

Empty

Empty adds an empty item. Empty items render nothing but are followed by a separator in lists.

c := Id("a").Op(":=").Id("b").Index(Lit(1), Empty())
fmt.Printf("%#v", c)
// Output:
// a := b[1:]

Line

Line inserts a blank line.

Clone

Be careful when passing *Statement. Consider the following...

a := Id("a")
c := Block(
	a.Call(),
	a.Call(),
)
fmt.Printf("%#v", c)
// Output:
// {
// 	a()()
// 	a()()
// }

Id("a") returns a *Statement, which the Call() method appends to twice. To avoid this, use Clone. Clone makes a copy of the Statement, so further tokens can be appended without affecting the original.

a := Id("a")
c := Block(
	a.Clone().Call(),
	a.Clone().Call(),
)
fmt.Printf("%#v", c)
// Output:
// {
// 	a()
// 	a()
// }

Cgo

The cgo "C" pseudo-package is a special case, and always renders without a package alias. The import can be added with Qual, Anon or by supplying a preamble. The preamble is added with File.CgoPreamble which has the same semantics as Comment. If a preamble is provided, the import is separated, and preceded by the preamble.

f := NewFile("a")
f.CgoPreamble(`#include <stdio.h>
#include <stdlib.h>

void myprint(char* s) {
printf("%s\n", s);
}
`)
f.Func().Id("init").Params().Block(
	Id("cs").Op(":=").Qual("C", "CString").Call(Lit("Hello from stdio\n")),
	Qual("C", "myprint").Call(Id("cs")),
	Qual("C", "free").Call(Qual("unsafe", "Pointer").Parens(Id("cs"))),
)
fmt.Printf("%#v", f)
// Output:
// package a
//
// import "unsafe"
//
// /*
// #include <stdio.h>
// #include <stdlib.h>
//
// void myprint(char* s) {
// 	printf("%s\n", s);
// }
// */
// import "C"
//
// func init() {
// 	cs := C.CString("Hello from stdio\n")
// 	C.myprint(cs)
// 	C.free(unsafe.Pointer(cs))
// }

File

Identifiers Keywords Operators Braces Parentheses Control flow Collections Literals Comments Generics Helpers Misc File

File represents a single source file. Package imports are managed automatically by File.

NewFile

NewFile Creates a new file, with the specified package name.

NewFilePath

NewFilePath creates a new file while specifying the package path - the package name is inferred from the path.

NewFilePathName

NewFilePathName creates a new file with the specified package path and name.

f := NewFilePathName("a.b/c", "main")
f.Func().Id("main").Params().Block(
	Qual("a.b/c", "Foo").Call(),
)
fmt.Printf("%#v", f)
// Output:
// package main
//
// func main() {
// 	Foo()
// }

Save

Save renders the file and saves to the filename provided.

Render

Render renders the file to the provided writer.

f := NewFile("a")
f.Func().Id("main").Params().Block()
buf := &bytes.Buffer{}
err := f.Render(buf)
if err != nil {
	fmt.Println(err.Error())
} else {
	fmt.Println(buf.String())
}
// Output:
// package a
//
// func main() {}

Anon

Anon adds an anonymous import.

f := NewFile("c")
f.Anon("a")
f.Func().Id("init").Params().Block()
fmt.Printf("%#v", f)
// Output:
// package c
//
// import _ "a"
//
// func init() {}

ImportName

ImportName provides the package name for a path. If specified, the alias will be omitted from the import block. This is optional. If not specified, a sensible package name is used based on the path and this is added as an alias in the import block.

f := NewFile("main")

// package a should use name "a"
f.ImportName("github.com/foo/a", "a")

// package b is not used in the code so will not be included
f.ImportName("github.com/foo/b", "b")

f.Func().Id("main").Params().Block(
	Qual("github.com/foo/a", "A").Call(),
)
fmt.Printf("%#v", f)

// Output:
// package main
//
// import "github.com/foo/a"
//
// func main() {
// 	a.A()
// }

ImportNames

ImportNames allows multiple names to be imported as a map. Use the gennames command to automatically generate a go file containing a map of a selection of package names.

ImportAlias

ImportAlias provides the alias for a package path that should be used in the import block. A period can be used to force a dot-import.

f := NewFile("main")

// package a should be aliased to "b"
f.ImportAlias("github.com/foo/a", "b")

// package c is not used in the code so will not be included
f.ImportAlias("github.com/foo/c", "c")

f.Func().Id("main").Params().Block(
	Qual("github.com/foo/a", "A").Call(),
)
fmt.Printf("%#v", f)

// Output:
// package main
//
// import b "github.com/foo/a"
//
// func main() {
// 	b.A()
// }

Comments

PackageComment adds a comment to the top of the file, above the package keyword.

HeaderComment adds a comment to the top of the file, above any package comments. A blank line is rendered below the header comments, ensuring header comments are not included in the package doc.

CanonicalPath adds a canonical import path annotation to the package clause.

f := NewFile("c")
f.CanonicalPath = "d.e/f"
f.HeaderComment("Code generated by...")
f.PackageComment("Package c implements...")
f.Func().Id("init").Params().Block()
fmt.Printf("%#v", f)
// Output:
// // Code generated by...
//
// // Package c implements...
// package c // import "d.e/f"
//
// func init() {}

CgoPreamble adds a cgo preamble comment that is rendered directly before the "C" pseudo-package import.

PackagePrefix

If you're worried about generated package aliases conflicting with local variable names, you can set a prefix here. Package foo becomes {prefix}_foo.

f := NewFile("a")
f.PackagePrefix = "pkg"
f.Func().Id("main").Params().Block(
	Qual("b.c/d", "E").Call(),
)
fmt.Printf("%#v", f)
// Output:
// package a
//
// import pkg_d "b.c/d"
//
// func main() {
// 	pkg_d.E()
// }

NoFormat

NoFormat can be set to true to disable formatting of the generated source. This may be useful when performance is critical, and readable code is not required.