Golang Quirks & Intermediate Tricks, Pt 1: Declarations, Control Flow, & Typesystem

A programming article by Efron Licht

Feb 2023

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On: Go, Programming Languages

Go is generally considered a ‘simple’ language, but it has more edge cases and tricks than most might expect.

You can be a productive go programmer without knowing about or using most or any of these tricks, but some of them are pretty handy. I’ll link to the go spec where appropriate throughout the article.

This is part 1 of what I hope to be a continuing series.

multi-statement lines with semicolons

Go is secretly a C-like language that terminates statements with semicolons. The semicolons are actually inserted early in compilation (during lexing). This means you can put multiple statements on the same line by inserting semicolons!

Be warned: gofmt will usually break them up into multiple lines. In fact, you can never have a single-line conditional. (Sorry, ternary conditional fans.)

Still, it can be handy for really small two-statement functions, like in tests:

func asJson(v any) []byte {b, _ := json.Marshal(v); return b}

methods as functions (“method expressions”)

Go methods are just functions. Given a type and method:

type Point struct{X, Y float64}
func (p Point) Add(q Point) Point{ 
    return Point{X: p.X+q.X, Y: p.Y+q.Y}
}

You can call the method in the ‘usual’ way by providing a receiver and using receiver.funcName(arg1, arg2, ...)

p, q := Point{1, 1}, Point{2,2}
fmt.Println(p.Add(q))

out: {3 3}

Or you can use the method as an ordinary, “bare” function via typeName.funcName(arg0, arg1, arg2)

p, q := Point{1, 1}, Point(2,2); 
fmt.Println(Point.Add(p, q))

out: {3, 3}

This is called a method expression. Unlike method calls, a method expression won’t automatically reference or de-reference a receiver for you, since there is no receiver.

That is, while this code compiles fine

import "math/big" // https://go.dev/play/p/-CHMNxIKumy
func main() {
 var x big.Float
 x.SetFloat64(10)
}

This code gives a compiler error

import "math/big" // https://go.dev/play/p/cv8TSURe15J
func main() {
 var x big.Float
 big.Float.SetFloat64(x, 10) // wrong
}

compiler error: invalid method expression big.Float.SetFloat64 (needs pointer receiver (*big.Float).SetFloat64)

The proper method expression is as follows

import "math/big" // https://go.dev/play/p/SRYxpp1UdVJ
func main() {
 var x big.Float
 (*big.Float).SetFloat64(&x, 10) // note parens
}

Method expressions don’t come up often, but they can occasionally save some work when you’re sorting or deduping.

select statements have break

select has no continue, but it does have break. This can lead to nasty bugs if you’re trying to break out of, say, an enclosing switch or loop. Use labels instead, as demonstrated by this code in the un-exported filelock package in go’s stdlib:

 // Wait until process Q has either failed or locked file B.
 // Otherwise, P.2 might not block on file B as intended.
locked:
 for {
  if _, err := os.Stat(filepath.Join(dir, "locked")); !os.IsNotExist(err) {
   break locked
  }
  select {
  case <-qDone:
   break locked
  case <-time.After(1 * time.Millisecond):
  }
 }

go can infer the type of composite literals in some contexts, but not others

The following code playground gives a terse and unhelpful compiler error:


func main() { // https://go.dev/play/p/CLu4AXg5qYW
 type Q struct{ A, B [3]int }
 structOfArrays := Q{{}, {}}
 fmt.Println(structOfArrays)
}

compiler error: missing type in composite literal

This implies that you always need to provide the types of composite literals, but that’s just not true. Go is happy to compile the following bode without me spelling out the type of each item on the right-hand side:

func main() { // https://go.dev/play/p/CLu4AXg5qYW
 type S struct{ N, M int }
 arrayOfStructs := [3]S{{}, {}, {0, 1}}
 fmt.Println(arrayOfStructs)
}

out: [{0 0} {0 0} {0 1}]

Or even this monstrosity:

func main() { // https://go.dev/play/p/kXLR8n7WdMc
 sliceOfMapOfArrayOfStructs := []map[string][2]struct{ N, M int }{{"foo": {{}, {M: 2}}}}
 fmt.Printf("%+v\n", sliceOfMapOfArrayOfStructs)
}

out: [map[foo:[{N:0 M:0} {N:0 M:2}]]]

The actual rule is this: go will infer the types of composite literals if they’re contained within an array, map, or slice, but struct fields and function arguments always need to spelled out explicitly.

There’s a long-open issue (#12584) hoping to address this inconsistency. I’d love to see more permissive composite literals.

simple expressions in switch statements

A switch statement can be proceeded by a simple statement:

// b is a *math/big.Int*
switch n, acc := b.Uint64(); acc {
    case big.Below:
        fmt.Println("< 0"),
    case big.Above:
        fmt.Println("> 18446744073709551615")
    case big.Exact:
        fmt.Println(n)
}

This works for expression switches and type switches:

switch a, err := f(); err.(type) {
}

If you omit the second part of the switch, you can do a “normal” boolean-value switch statement:

// some kind of low-level networking call:
var try int
var packets []Packet
READ:
for {
    switch packet, err := readPacket(ctx, conn, buf);  { // note semicolon
        case errors.Is(err, io.EOF): 
            packets = append(packets, packet)
            break READ
        case err == nil:
            packets = append(packets, packet)
            try = 0
        case errors.As(err, fatalErr) || try == maxTries:
            return fmt.Errorf("fatal error after %d retries: %v", i, err)
        default:
            const wait = 100*time.Millisecond
            log.Printf("error: retrying in %d", wait)
            try++
            time.Sleep(wait)
    }
}

I like the look of these: they allow very terse, expressive code, but they’re rare & unusual enough to probably cause confusion. Most of the time you’re better off with a chain of if.

GOTO exists

The oft-maligned GOTO is an excellent piece of kit. Go’s GOTO is somewhat limited: you can’t jump into a new block or out of a function, so it’s hard to get yourself into the kind of trouble you could in 1980s BASIC.

This means you can’t do something like this, since you’ll get a compiler error:


func main() { // https://go.dev/play/p/1krGFE6FvgJ
 goto label

 if true {
  v := 3
  panic(v)

 label:
  fmt.Println(v) // what's the value of v?
 }
}

compiler error: ./prog.go:11:7: goto label jumps into block starting at ./prog.go:13:10

Speaking of which:

you can make a block at any time

You don’t need an if, for, func, or any other keyword to make a block.

{
    name := "efron"
    {
        fmt.Println("hi ", name)
    }
}

I find this useful for complicated variable initialization. Here’s an example from the fmtbench tool I wrote in the last article

// fmtbench.go
// context: the variable sortBy is a command-line flag specifying the sort order. we've already validated it.
// results is a []struct{
//     name                    string,
//  runs, ns, bytes, allocs float64
// }
{
 // sort results
  var less func(i, j int) bool
  switch *sortBy {
  default:
    goto PRINT
  case "allocs":
   less = func(i, j int) bool { return results[i].allocs < results[j].allocs }
  case "name":
   less = func(i, j int) bool { return results[i].name < results[j].name }
  case "runtime":
   less = func(i, j int) bool { return results[i].ns < results[j].ns }
  }
  sort.Slice(results, less)
 }
PRINT:
 for _, res := range results {
  fmt.Printf("|%s|%.3g|%.3g|%0.3g|%.3g|%0.3g|%.3g|%0.3g|\n", res.name, res.runs, res.ns, (res.ns/maxNS)*100, res.bytes, (res.bytes/maxBytes)*100, res.allocs, (res.allocs/maxAllocs)*100)
 }

By using a block here, we make it immediately clear that less is only going to exist for the context of this sort

We could make a function for this, but that means jumping around, for us, the compiler, and the runtime (assuming it’s not inlined).

Try starting with blocks, and promote them to functions when you find yourself needing to re-use the code.

But sometimes you do need a function, even for a single use:

immediately-evaluated-function-expressions

You can define a function and invoke it on the same line:

// playground: https://go.dev/play/p/dmNloKFUGSZ
package main

import (
 "crypto/rand"
 "encoding/binary"
 "fmt"
)

var seed uint64 = func() uint64 {
 var b = make([]byte, 8)
 _, _ = rand.Read(b)
 return binary.LittleEndian.Uint64(b)
}()

func main() {
 fmt.Println(seed)
}

This is the catchily-named “immediately-evaluated-function-expression”, or IIFE for short. These are invaluable in languages which privelege functions over other kinds of blocks: for example, Javascript before it got the let keyword had no block scope, so you had to define functions every time you wanted a new namespace.

Go privleges functions over blocks in two ways:

There’s basically only two uses for IIFE’s instead of blocks:

If you’re going to use an IIFE with a return value, use the var declaration instead of := - it makes it easier for the reader to understand the flow. And don’t overdo it - you can always just define a closure and call it on the next line.

you can declare types inside blocks

You can declare types inside any kind of block, but you can’t declare methods on those types. You can define a function that takes that type using a function expression (“closure”).

import "fmt"
func main() { // playground: https://go.dev/play/p/vAkgOTnEg7d
 type Point struct{ X, Y float64 }
 addPoint := func(p, q Point) Point { return Point{p.X + q.X, p.Y + q.Y} }
 q := addPoint(Point{2, 3}, Point{-1, 1})
 fmt.Println(q)
}

output: {1 4}

This obeys the ordinary block-scope rules, so this would be a compiler error:

func main() { // https://go.dev/play/p/_ytvmPewLTA
    {
        type Point struct{X, Y float64}
    }
    var p Point 
}

compiler error: ./prog.go:9:8: undefined: Point

This can make your code more straightforward. Just like variables, it’s best to define a type as close to it’s use and with as small of a scope as possible.

go has anonymous structs

Sometimes you don’t have to declare the type at all: go allows anonymous struct values. This is especially handy for functions like json.Marshal and json.Unmarshal which just depend on the shape of the type.

These anonymous structs can nest:

func main() { // https://go.dev/play/p/vA5SJ-GKJMm
 var s struct{ Name struct{ First, Last string } }
 json.Unmarshal([]byte(`{"name": {"first": "efron", "last": "licht"}}`), &s)
 fmt.Printf("%+v\n", s)
}

output: {Name:{First:efron Last:licht}}

You can even make custom struct tags for your individual use case:

// GET /health
import "json"
import "net/http"
func WriteHealth(w http.ResponseWriter, _ *http.Request) {
    json.NewEncoder(w).Encode(struct {
        Uptime time.Duration `json:"uptime"`
        Stats struct {
            Hits int64 `json:"hits"`
            Misses int64 `json:"misses"`
            Errors int64 `json:"errors"`
        } `json:"stats"`
    })
}

… and anonymous interfaces

You never have to declare the type of an interface: anywhere you can use io.Writer, you can use interface{Write([]byte)(int, error)}.

This can be handy for runtime specialization (that is, when you want to check if a type fulfills extra interfaces)

import "gzip"
func writeZipped(w io.Writer, b []byte) (int, error) {
    zipw := gzip.NewWriter(w)
    n, err := zipw.Write(w)
    if err != nil {
        return n, err
    }
    if err := zipw.Close(); err != nil {
        return err
    }
    // flush the underlying buffer, if there is one
    if f, ok := w.(interface{Flush() error}); ok {
        _ = f.Flush()
    }
    // sync to disk if possible
    if f, ok := w.(interface{Sync() error}); ok {
        _ = f.Sync()
    }
}

This is especially useful for function signatures. Suppose I’m going to call out to a database as part of a function.

(An aside: I don’t particularly like mocking: I’d love to write an article about strategies you can use to avoid it).

The ‘ordinary’ function signature would look something like this:

func SelectUser(ctx context.Context, db *sql.DB, userID uuid.UUID) (username string, createdAt time.Time, err error) {
    const query = `SELECT username, created_at FROM users where user.id = $1;`
    db.QueryRowContext(ctx, query, userID).Scan(&username, createdAt)
}

Suppose at some point we need to mock this for a test. *sql.DB is a struct, and it’s not immediately apparent what we’d call the interface we’d replace it with. DBer? QueryRowContexter? In this case, we can be clearest by omitting the name entirely: all the reader needs to know is that the DB has a function that looks like QueryRowContext().

We can make this mockable by just changing the function signature to use an anonymous interface.

func SelectUser(
    ctx context.Context, 
    db interface{QueryRowContext(context.Context, string, ...args) *sql.Row},
    userID uuid.UUID
) (username string, createdAt time.Time, err error) {

I think the anonymous interface is actually clearer than the named one for most single-method interfaces.

Both anonymous structs and interfaces can be generic, too.

zero-sized type (“ZST”)

The empty struct struct{} and arrays of length zero (like [0]int) take up no memory, as do structs and arrays comprised entirely of zero-sized types.

A zero-sized type (“ZST”) is most often used as an interface handle, like io.Discard.


## zero-sized types
// io/io.go

// Discard is a Writer on which all Write calls succeed
// without doing anything.
var Discard Writer = discard{}
type discard struct{}

func (discard) Write(p []byte) (int, error) { return len(p), nil}
func (discard) WriteString(s string) (int, error) { return len(s), nil}

You can also use a ZST as a map value type to save space rather than using map[string]bool

var set = make(map[string]struct{})

but don’t: map[string]bool is just as fast and has a much cleaner api.

You can get kind of silly with this:

type cursedZST = [0]map[struct{}]struct{} // don't do this.

Zero-sized types have a third use, but we’ll need to talk about blank struct fields first.

blank struct fields

Struct types can have unreachable fields using the blank identifier, _ as the field name. You can use blank fields:

Putting it together: A generic zero-sized type

As weird as it sounds, I have a use for a zero-sized, generic struct with unreachable members:
context.WithValue.

Let’s review the documentation:

func WithValue(parent Context, key, val any) Context

WithValue returns a copy of parent in which the value associated with key is val. Use context Values only for request-scoped data that transits processes and APIs, not for passing optional parameters to functions. The provided key must be comparable and should not be of type string or any other built-in type to avoid collisions between packages using context. Users of WithValue should define their own types for keys. To avoid allocating when assigning to an interface{}, context keys often have concrete type struct{}. Alternatively, exported context key variables’ static type should be a pointer or interface.

Most request-scoped data is a singleton per request. That is, it doesn’t make sense for a request to carry around multiple loggers, users, traces; you want to carry the same one with you from function call to function call

The usual way Go programs have handled this is by making a separate context key per type you want to carry in the struct. But with the advent of generics in go1.18, instead of having to make a new zero-sized type for every struct, we can just make a single generic zero-sized type and use it for everything:

type key[T] struct{}
// FromCtx returns the value of type T stored in the context, if any:
func FromCtx[T](ctx context) (T, bool) { 
    t, ok := context.Value(key[T]{}).(T)
    return t, ok
}
// WithValue returns a copy of parent in which the value associated with `CtxKey[T]{}` is
// val.
func WithValue[T](ctx context, t T)(context.Context) {
    return context.WithValue(ctx, key[T]{}, t)
}

For fun, let’s rewrite FromCtx as a truly hellish one-liner using (nearly) every trick we’ve learned so far:

func FromCtx[T any](ctx context.Context) (T, bool) {t, ok := context.Context.Value(ctx, [0]struct{_ T}).(T);return t, ok}

That’s right: this ugly SOB has a

… please don’t do this.

Next time(?)

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