TryGo: Go with 'try' operator

This is a translator of 'TryGo' as my experiment to see what happens if Go were having try() function. Basic idea of try() came from Rust's try! macro (or ? operator). try() handles if err != nil check implicitly.

This package provides a code translator from TryGo (Go with try()) to Go.

Go:

func CreateFileInSubdir(subdir, filename string, content []byte) error {
    cwd, err := os.Getwd()
    if err != nil {
        return err
    }

    if err := os.Mkdir(filepath.Join(cwd, subdir)); err != nil {
        return err
    }

    p := filepath.Join(cwd, subdir, filename)
    f, err := os.Create(p)
    if err != nil {
        return err
    }
    defer f.Close()

    if _, err := f.Write(content); err != nil {
        return err
    }

    fmt.Println("Created:", p)
    return nil
}

TryGo:

func CreateFileInSubdir(subdir, filename string, content []byte) error {
    cwd := try(os.Getwd())

    try(os.Mkdir(filepath.Join(cwd, subdir)))

    p := filepath.Join(cwd, subdir, filename)
    f := try(os.Create(p))
    defer f.Close()

    try(f.Write(content))

    fmt.Println("Created:", p)
    return nil
}

There is only one difference between Go and TryGo. Special magic function try() is provided in TryGo.

Spec

try looks function, but actually it is a special operator. It has variadic parameters and variadic return values. In terms of Go, try looks like:

func try(ret... interface{}, err error) (... interface{})

Actually try() is a set of macros which takes one function call and expands it to a code with error check. It takes one function call as argument since Go only allows multiple values as return values of function call.

In following subsections, $zerovals is expanded to zero-values of return values of the function. For example, when try() is used in func () (int, error), $zerovals will be 0. When it is used in func () (*SomeStruct, SomeInterface, SomeStruct, error), $zerovals will be nil, nil, SomeStruct{}.

Implementation:

• Definition statement
• Assignment statement
• Call statement
• Call Expression

Definition statement

$Vars := try($CallExpr)

var $Vars = try($CallExpr)

Expanded to:

$Vars, err := $CallExpr
if err != nil {
    return $zerovals, err
}

var $Vars, err = $CallExpr
if err != nil {
    return $zerovals, err
}

Assignment statement

$Assignee = try($CallExpr)

Expanded to:

var err error
$Assignee, err = $CallExpr
if err != nil {
    return $zerovals, err
}

Assignment operation x op= y (e.g. x += y) is supported.

$Assignee op= try($CallExpr)

Expanded to:

$tmp, err := $CallExpr
if err != nil {
    return $zerovals, err
}
$Assignee op= $tmp

Call statement

try($CallExpr)

Expanded to:

if $underscores, err := $CallExpr; err != nil {
    return err
}

$underscores, is a set of _s which ignores all return values from $CallExpr. For example, when calling func() (int, error), it is expanded to _. When calling func() (A, B, error) in try(), it is expanded to _, _. When calling func() error in try(), it is expanded to an empty.

Call Expression

try() call except for toplevel in block

1 + try($CallExpr)

Expanded to:

$tmp, err := $CallExpr
if err != nil {
    return $zerovals, err
}
1 + $tmp

This should allow nest. For example,

1 + try(Foo(try($CallExpr), arg))

$tmp1, err := $CallExpr
if err != nil {
    return $zerovals, err
}
$tmp2, err := Foo($tmp1, arg)
if err != nil {
    return $zerovals, err
}
1 + $tmp2

The order of evaluation must be preserved. For example, when try() is used in a slice literal element, elements before the element must be calculated before the if err != nil check of the try().

For example,

ss := []string{"aaa", s1 + "x", try(f()), s2[:n]}

will be translated to

tmp1 := "aaa"
tmp2 := s1 + "x"
tmp3, err := f()
if err != nil {
    return $zerovals, err
}
ss := []string{tmp1, tmp2, tmp3, s2[:n]}

Ill-formed cases

• try() cannot take other than function call. For example, try(42) is ill-formed.
• try() is expanded to code including return. Using it outside functions is ill-formed.
• When function called in try() invocation does not return error as last of return values, it is ill-formed.

These ill-formed code should be detected by translator and it will raise an error.

Why try() 'function'? Why not ? operator?

Following code may look even better. At least I think so.

func CreateFile(subdir, filename string, content []byte) error {
    cwd := os.Getwd()?
    os.Mkdir(filepath.Join(cwd, subdir))?
    f := os.Create(filepath.Join(cwd, subdir, filename))?
    defer f.Close()
    f.Write(content)?
    return nil
}

The reason why I adopted try() function is that ...TODO

Installation

Download an executable binary from release page (NOT YET).

To build from source:

$ go get -u github.com/rhysd/trygo/cmd/trygo

Usage

$ trygo -o {outpath} {inpaths}

{inpaths} is a list of directory paths of Go packages you want to translate. The directories are translated recursively. For example, when dir is passed and there are 2 packages dir and dir/nested, both packages will be translated.

{outpath} is a directory path where translated Go packages are put. For example, when dir is specified as {inpaths} and out is specified as {outpath}, dir/** packages are translated as out/dir/**.

License

MIT License