- go-for-perl-hackers
- Your editor
- Go vs Perl
- Comments
- Variables
- Lists
- Dumping Data Structures
- File Operations
- Flow Control
- Loops
- Functions
- Running Tests
- Debugging
- Sleep
- Parsing URIs
- Changing URI Query Params
- Command Line Scripts
go-for-perl-hackers
The primary aim of this cheat sheet is to help Perl programmers get up and running with Go.
Your editor
vim
Consider adding the vim-go
plugin to your .vimrc
If you're using vim-plug, that would look something like:
Plug 'fatih/vim-go'
If you're using Pathogen, that would look something like:
runtime bundle/vim-pathogen/autoload/pathogen.vim
" Bundle: tpope/vim-pathogen
call pathogen#infect()
" Bundle: https://github.com/fatih/vim-go.git
Now, open vim
after installing vim-go
and enter :GoInstallBinaries
Also, you can add goimports
:
First install it: go get golang.org/x/tools/cmd/goimports
Then add the following to your .vimrc
let g:go_fmt_command = "goimports"
If your editor is set up to display tabs visually, you may want to disable that for Go files.
autocmd FileType go setlocal nolist
shortcuts
daf
in vim
will now allow you to cut an entire function without first
needing to select it.
vim-go commands
:GoBuild
go build
:GoRun
go run
:GoGenerate
go generate
:GoRename
Call this when your cursor is over something you'd like to rename, and it will be renamed in all of the appropriate locations.
:GoTest
Run this in order to test your code without leaving your vim
session.
:GoTestFunc
Run this when inside a test function and only this test function will be run. Equivalent of go test -run MyFunctionName
.
:GoAlternate
Toggles file between foo.go
and foo_test.go
:GoDef
Takes you to the location where an item is defined. Keep running this to move further up the stack.
:GoDefPop
Pops the stack which you created with :GoDef
, taking you back to the previous location(s) you called :GoDef
from.
:GoDecls
See functions which are declared in the current file.
:GoDeclsDir
Like :GoDecls
but finds all the functions declared in the directory of the file you're editing.
:GoReferrers
Find other places where a function or variable is being invoked.
:GoDoc
Look up docs for a function.
:GoInfo
Get a function's input and output parameters.
:GoDescribe
Like :GoInfo
, but with more info.
:GoFiles
List all files in package.
:GoDeps
List dependencies.
:GoWhicherrs
Get info on what sorts of errors an err
variable may contain.
:GoCallers
Find out where a function callers are.
:GoImpl
Generate stub methods required by an interface.
:GoFreevars
Go vs Perl
In this section we'll document some commonly used Perl constructs and try to find their equivalents in Go.
Comments
Perl:
# single line
=pod
Multi line
=cut
Go:
// single line (C++-style)
/*
Multi-line (C-Style)
*/
Printing strings
Perl:
print 'hello, world';
Go:
package main
import "fmt"
func main () {
fmt.Print("hello, world")
}
Formatted print statements.
Perl:
printf('We are open %i days per %s', 7, 'week');
Go:
package main
import ( "fmt" )
func main() {
fmt.Printf("We are open %d days per %s", 7, "week")
}
Printing from within a test
Perl:
diag 'foo happens';
Go:
t.Log("foo happens")
t.Logf("We are open %d days per %s", 7, "week")
Variables
Environment Variables
Perl:
$ENV{FOO} = 'bar';
local $ENV{FOO} = 'bar'; # Same as above, but with local scope
print "GOPATH: $ENV{GOPATH}\n";
Go:
os.Setenv("FOO", "bar")
fmt.Println("GOPATH: ", os.Getenv("GOPATH"))
Variable Assignment
Perl:
my $foo = 'bar';
my $pi = 3.14;
my $no_assignment;
Go:
// the following assignments are equivalent
var foo = "bar"
foo := "bar"
var pi float32 = 3.14 // explicit cast as float32
pi := float32(3.14) // explicit cast as float32
pi := 3.14 // implicit cast as float64
pi := "3.14" // implicit cast as string
var noAssignment string // equivalent to: noAssignment := ""
See golang.org/ref/spec#Rune_literals
Multiple Variables
Declare without explicit values
Perl:
my ($foo, $bar);
Go:
var foo, bar int
var nothing []string // create an empty slice
Declare with explicit values
Perl:
my ($i, $j) = (1, 2);
Go:
var i, j int = 1, 2
Double vs Single Quotes
Perl:
my $foo = 'bar'; # no variable interpolation
my $bar = "$foo baz"; # allow for variable interpolation
Go:
foo := "本" // implicitly cast as a string
foo := '本' // implicitly cast as a rune
Multiline strings
Perl:
my $long_string = <<'EOF';
my multiline
string
EOF
Use double quotes <<"EOF";
if you need to interpolate variables.
Go:
longString := `
my multiline
string
`
Boolean checks (true/false)
Perl:
my $success = 1; # true
$success = 'foo'; # true
$success = 0; # false
$success; # (undef) false
if ($success) {
print "This succeeded";
}
if ( !$success ) {
print "This failed";
}
Go:
var success bool
success = true
success = false
if success == true {
fmt.Println("This succeeded")
}
if success {
fmt.Println("This succeeded")
}
if success == false {
fmt.Println("This failed")
}
if !success {
fmt.Println("This failed")
}
Checking for (un)definedness
Perl:
my $foo;
if ( ! defined $foo ) {
...;
}
Go:
var myString string
if myString == "" {
fmt.Println("Empty")
}
var mySlice []int
if mySlice == nil {
fmt.Println("nil")
}
Incrementing and Decrementing Integer
Perl:
See https://perldoc.perl.org/perlop.html#Auto-increment-and-Auto-decrement
$i = 0; $j = 0;
print $i++; # prints 0
print ++$j; # prints 1
$i = 0; $j = 0;
print $i--; # prints 0
print --$j; # prints -1
Go:
counter := 1
counter++
counter--
String Concatenation
Perl:
my $foo = 'go';
my $bar = 'pher';
$gopher = "$foo$bar";
$gopher = $foo . $bar;
$gopher = join q{}, $foo, $bar;
$gopher = sprintf '%s%s', $foo, $bar;
Go:
package main
import (
"fmt"
"strings"
)
func main() {
var gopher string
gopher = foo + bar
gopher = fmt.Sprintf("%s%s", foo, bar)
}
package main
import (
"fmt"
"strings"
)
func main() {
foo := "go"
bar := "pher"
gopher = strings.Join([]string{"go", "pher"}, "")
fmt.Println(gopher)
}
package main
import (
"bytes"
"fmt"
)
func main() {
var buffer bytes.Buffer
foo := "go"
bar := "pher"
buffer.WriteString(foo)
buffer.WriteString(bar)
fmt.Println(buffer.String())
}
Constants
Perl:
use Const::Fast;
const my $hello => 'Hello, world';
Go:
// Create an *untyped* string constant
const hello = "Hello, world"
// Create a typed string constant
const hello string = "Hello, World"
Create multiple constants with one const
declaration:
const(
hello = "Hello, world"
goodbye = "Goodbye!"
)
Constants cannot be declared using the := syntax.
Lists
Create an Array
Perl:
my @foo = (1..3);
my $first = $foo[0];
Go:
foo := [3]int{1,2,3}
first := foo[0]
Note that creating an empty array in Go means that it will be populated by the type's default values:
var bar [5]int // creates an array of [0,0,0,0,0]
Size of an array:
Perl:
my $size = @array;
Go:
size := len(array)
Hashes / Structs
Perl:
use Data::Printer; # exports p()
my %foo = (
X => 1,
Y => 2,
);
$foo{X} = 4;
print $foo{X}; # prints 4
p %foo;
# prints:
# {
# X => 4,
# Y => 2,
# }
delete $foo{X};
Go:
package main
import "fmt"
type Vertex struct {
X int
Y int
}
func main() {
v := Vertex{1, 2}
v.X = 4
fmt.Println(v.X) // prints 4
fmt.Printf("%+v\n", v) // prints {X:4 Y:2}
// additional examples
v1 = Vertex{1, 2} // has type Vertex
v2 = Vertex{X: 1} // Y:0 is implicit
v3 = Vertex{} // X:0 and Y:0
delete(v, "X")
}
Iterating Over a List
Perl:
my @foo = ('foo', 'bar', 'baz');
for my $value ( @foo ) {
print "$value\n";
}
my @foo = ('foo', 'bar', 'baz');
for (@foo) {
print "$_\n";
}
// Print array index for each element
my @foo = ('foo', 'bar', 'baz');
for my $i (0..$#foo) {
print "$i: $foo[$i]\n";
}
Go:
package main
import (
"fmt"
)
func main() {
foo := [3]int{1, 2, 3}
for i, v := range foo {
fmt.Printf("index: %v value: %v\n", i, v)
}
}
Splitting a string
my @list = split ',', 'a,b,c'
package main
import (
"strings"
)
mySlice := strings.Split("a,b,c", ",")
Iterating Over a Hash/Map
Perl:
my %hash = ( key_1 => 'foo', key_2 => 'bar', );
for my $key ( keys %hash ) {
printf( "key: %s value: %s\n", $key, $hash{$key} );
}
Go:
package main
import (
"fmt"
)
func main() {
myMap := map[string]string{"key1": "foo", "key2": "bar"}
for k := range myMap {
fmt.Printf("key: %s value: %s\n", k, myMap[k])
}
}
Checking if a Hash/Map Key Exists
Perl:
my %pages = ( home => 'https://metacpan.org' );
if ( exists $foo{home} ) {
...
}
Go:
https://stackoverflow.com/a/2050629/406224
package main
import "fmt"
func main() {
pages := make(map[string]string)
pages["home"] = "https://metacpan.org"
if _, ok := pages["home"]; ok {
fmt.Println("ok")
}
}
Deleting a Hash/Map Key
Perl:
my %pages = ( home => 'https://metacpan.org' );
delete $pages{home};
Go:
package main
import "fmt"
func main() {
pages := make(map[string]string)
pages["home"] = "https://metacpan.org"
delete(pages, "home")
}
Getting an array Hash/Map Keys
Perl:
my %pages = ( home => 'https://metacpan.org' );
my @keys = keys %pages;
Go:
package main
func main() {
pages := make(map[string]string)
pages["home"] = "https://metacpan.org"
keys := []string{}
keys = append(keys, "foo")
}
Slices:
Perl:
my @array = (0..5);
my @slice = @list[2..4];
Go:
array := [6]int{0,1,2,3,4,5}
var slice []int = array[2:4]
var myslice []int // create an empty slice of integers
var nothing []string // create an empty slice of strings
Note that arrays in Go have a fixed size, whereas slices are dynamically sized.
Also:
A slice does not store any data, it just describes a section of an underlying array.
Changing the elements of a slice modifies the corresponding elements of its underlying array.
Other slices that share the same underlying array will see those changes.
See https://tour.golang.org/moretypes/8
Note also that slices in Go can use defaults for lower and upper bounds. That means that for the array of 10 integers var a [10]int
, the following slices are equivalent:
a[0:10] // explicit lower to upper bound
a[:10] // use default lower bound (0)
a[0:] // use default upper bound (0)
a[:] // use default upper and lower bounds (0 and 10)
Note that the lower bound is the starting point in the index (ie 0) and the length of the slice is the upper bound, which is why the entire slice consists of a[0:10
and not a[0:9]
.
See https://tour.golang.org/moretypes/10
Appending Slices:
Perl:
my @array = (0..5);
my @slice = @list[2..4];
push @slice, 11, 12;
Note that in Perl, a slice of an array is also an array, so there's no need to make a distinction between the two.
Go:
array := [6]int{0,1,2,3,4,5}
var slice []int = array[2:4]
slice = append(slice, 11, 12)
Dumping Data Structures
To your terminal
Perl:
use strict;
use warnings;
use feature qw( say );
use Data::Printer; # exports p() and np()
my %foo = ( a => 'b' );
p( %foo );
# or
say np( %foo );
Go:
package main
import "fmt"
func main() {
var config struct {
user string
pass string
}
config.user = "florence"
config.pass = "machine"
fmt.Printf("%+v", config)
return
}
Or:
package main
import "github.com/davecgh/go-spew/spew"
func main() {
var config struct {
user string
pass string
}
config.user = "florence"
config.pass = "machine"
spew.Dump(config)
return
}
To disk (write)
Perl:
use Data::Printer; # exports np()
use Path::Tiny qw(path);
my @list = ( 1..3 );
path('/tmp/foo.txt')->spew( np( @list ) );
Go:
package main
import (
"log"
"os"
"github.com/davecgh/go-spew/spew"
)
func main() {
list := [3]int{1, 2, 3}
file, err := os.OpenFile(
"/tmp/foo.txt",
os.O_CREATE|os.O_WRONLY,
0666,
)
if err != nil {
log.Fatal(err)
}
spew.Fdump(file, list)
file.Close()
}
To disk (append)
Perl:
use Data::Printer; # exports np()
use Path::Tiny qw(path);
my @list = ( 1..3 );
path('/tmp/foo.txt')->append( np( @list ) );
Go:
package main
import (
"log"
"os"
"github.com/davecgh/go-spew/spew"
)
func main() {
list := [3]int{1, 2, 3}
file, err := os.OpenFile(
"/tmp/foo.txt",
os.O_APPEND|os.O_CREATE|os.O_WRONLY,
0666,
)
if err != nil {
log.Fatal(err)
}
spew.Fdump(file, list)
file.Close()
}
File Operations
Creating a directory
Perl:
use Path::Tiny qw( path );
use Try::Tiny qw( catch try );
my $dir = '.my-perl-cache';
try {
path($dir)->mkpath( { chmod => 0644 });
}
catch {
die sprintf "Could not create dir %s because of %s", $dir, $_;
};
Go:
package main
import (
"log"
"os"
)
func main() {
cacheDir := ".my-go-cache"
if err := os.MkdirAll(cacheDir, 0644); err != nil {
log.Printf("Cannot create dir %s because %v", cacheDir, err)
}
}
Read an Entire File
Perl:
use Path::Tiny qw( path );
my $content = path('path', 'to', 'file')->slurp_utf8;
Go:
Note that in this case content
is []byte
content, err := ioutil.ReadFile(filepath.Join("path", "to", "file"))
if err != nil {
log.Fatal(err)
}
// convert byte array to string
contentAsString := string(content[:])
Read First Line of a File
Perl:
In the Perl example, we'll chomp the line to make explicit that newlines need to be handled.
use Path::Tiny qw( path );
my $first_line;
my $fh = path('/path/to/file')->openr_utf8;
while (my $line = <$fh>) {
$first_line = $line;
chomp $first_line;
last;
}
print $first_line, "\n";
Go:
scanner.Scan()
helpfully trims newlines for us.
import(
"fmt"
"log"
)
func main() {
file, err := os.Open("/path/to/file")
if err != nil {
log.Fatal(err)
}
defer file.Close()
scanner := bufio.NewScanner(file)
var firstLine string
for scanner.Scan() {
firstLine = scanner.Text()
break
}
if err := scanner.Err(); err != nil {
log.Fatal(err)
}
fmt.Println(firstLine)
}
Flow Control
if
Perl:
if ( $foo > 1 ) {
print 'bar';
}
Go:
if ( foo > 1 ) {
fmt.Println("bar")
}
// parens are optional
if foo > 1 {
fmt.Println("bar")
}
else
Perl:
if ( $foo > 1 ) {
print 'bar';
}
else {
print 'baz';
}
Go:
if foo > 1 {
fmt.Println("bar")
} else {
fmt.Println("baz")
}
elsif / else if
Perl:
if ( $foo > 1 ) {
print 'bar';
}
elsif ( $foo < 10 ) {
print 'baz';
}
Go:
if foo > 1 {
fmt.Println("bar")
} else if foo < 10 {
fmt.Println("baz")
}
Loops
For loops
Perl:
my $sum;
for ( my $i = 0 ; $i < 10 ; $i++ ) {
$sum += $i;
}
Go:
sum := 0
for i := 0; i < 10; i++ {
sum += i
}
While loops
Perl:
my $sum = 0;
my $i = 0;
while ( $i < 10 ) {
$sum += $i++;
}
Go:
// The init and post statement in a Go for loop are optional.
sum := 0
i := 0
for i < 10 {
sum += i
i += 1
}
Infinite loops
Perl:
while (1) {
}
Go:
for {
}
Short-circuiting a loop iteration
Perl:
while (1) {
...
next if $foo eq 'bar';
...
}
Go:
for {
if foo == "bar" {
continue
}
// Won't get here if continue is called
}
Note that continue
will immediately begin the next iteration of the innermost for
loop.
Terminating a loop
Perl:
while (1) {
...
last if $foo eq 'bar';
...
}
Go:
for {
if foo == "bar" {
break
}
// Won't get here if break is called
}
Note that break
will exit the enclosing loop at the point where it is called.
Today's Date as YYYY-MM-DD
Perl
use DateTime ();
print DateTime->now->ymd;
Go
https://play.golang.org/p/k0ijssDDOU6
package main
import (
"fmt"
"time"
)
func main() {
fmt.Println(time.Now().Format("2006-01-02"))
}
Functions
Functions without signatures
Perl:
sub foo {
print 'ok';
}
foo();
Go:
package main
import "fmt"
func main() {
foo()
}
func foo() {
fmt.Println("foo")
}
Running Tests
Perl:
$ perl Makefile.PL
$ make
$ make test
or
$ prove -l t/path/to/test.t
Use the -v
flag for verbose output:
$ prove -lv t/path/to/test.t
Go:
$ go test
Use the -v
flag for verbose output:
$ go test -v
If you're using vim-go
, use :GoTest
either from your foo.go
or
foo_test.go
. (Note, you can also use :GoAlternate
to toggle between the
two files.)
To test a subset of functions:
$ go test -run regexp
If you're using vim-go
, move your cursor to the name of the function you'd
like to test. Running :GoTest
here will run the function you're currently
in.
To bypass Go's test caching:
$ go test -count=1
Debugging
Printing Stack Traces
Perl:
use Carp qw( longmess );
print longmess();
Go:
package main
import (
"runtime/debug"
)
func main() {
debug.PrintStack()
}
Sleep
Perl:
sleep 60;
Go:
package main
import (
"time"
)
func main() {
time.Sleep(60 * time.Second)
}
Parsing URIs
Perl:
use Mojo::URL ();
my $url = Mojo::URL->new('https://www.google.com/search?q=schitt%27s+creek');
print $url->query->param('q'); # schitt's creek
Go:
import (
"fmt"
"log"
"net/url"
)
func main() {
url, err := url.Parse("https://www.google.com/search?q=schitt%27s+creek")
if err != nil {
log.Fatal(err)
}
q := url.Query()
fmt.Println(q.Get("q")) // schitt's creek
}
Changing URI Query Params
Go:
package main
import (
"fmt"
"net/url"
)
func main() {
url, _ := url.Parse("https://example.com")
q := url.Query()
q.Set("activity", "dance")
q.Set("type", "flash")
url.RawQuery = q.Encode()
fmt.Println(url)
}
Command Line Scripts
Print first argument to a script
print $ARGV[0], "\n" if $ARGV[0];
package main
import (
"fmt"
"os"
)
func main() {
if len(os.Args) > 1 {
fmt.Printf("%v\n", os.Args[1])
}
}
Exiting a script
Perl:
exit(0);
Go:
import("os")
os.Exit(0)