Mechanical

A language that makes building webapps (and more!) as easy as building finite-state machines.

• Best of both worlds between declarative and imperative: Just say what you want to happen instead of how, by declaratively defining your program as a state machine (like The Elm Architecture). Unlike Elm, straightforwardly perform side-effects using imperative-like yet referentially transparent syntax.
• X% faster and Y% smaller than React: Instead of virtual DOM diffing, just compile to imperative, mutative JavaScript.
• Static types, no type annotations: The benefits of static typing like autocomplete and compile-time error checking, never write a single type annotation.
• Fully interoperable: Compile to JavaScript modules that can call or be called by any JS library or browser/Node API.

Status: First compile and run of Hello World works—and absolutely nothing else. Help wanted!

Example

Mechanical source code:

State counter = 0

-- JSX-like view syntax:
View: "#app" <div>
    <p>{counter}</p>
    <button @increment>+1</button>
</div>

-- Event handler:
When receive increment.Click:
  Change counter to counter + 1

Compiles to clean, readable JavaScript:

render_view("#app", <div>
    <p>{counter}</p>
    <button {...{ "mech-name": "increment" }}>+1</button>
</div>)

const increment = {
    Click: event_stream("increment", "click"),
}

increment.Click.subscribe(() => {
    counter += 1
    p.textContent = counter
})

Usage

Install:

npm install mechanical

Run the compiler:

npx mechc some_file.mech

Which will output: some_file.js

Differences from JS

Extensions

Mechanical-specific extensions to JS expression syntax (which only make sense with Mechanical semantics, and wouldn't make sense for JS:

• (TODO) Cmd { ... }

• (TODO) Hashtagged values #tag value and pattern-matching match tagged {...}

  Let x = Current switch_state ? #on 72 : #off "sleeping for the night"
  // expression-form pattern-matching:
  Let y = match x { #on temp -> temp; #off message -> 68 }
  // statement-form pattern-matching:
  Match x:
      #on temp ->
          Change color to #green
          Change temperature_dial to temp
      #off message ->
          Change display_message to message.slice(from: 0, to: 100)

Extensions to JavaScript features:

• Multiline strings

  • Both string literals ("" and '') can have newlines in them, but subsequent lines must be indented to at least the same level as the open-quote; that indentation is then omitted from the result:

    When button.Click:
        Let valid_string = "first line
        second line
          third line" // == "first line\nsecondline\n  third line"
    
    When button2.Click:
        Let invalid_string = "first line
      second line" // error!

• (TODO) Alternate string interpolation syntax $`text {expression} text`

  • In addition to JS template literal syntax (e.g. `text ${expression} text`), Mechanical supports an alternative string interpolation syntax where you prefix the string with $:

    $`text {expression} text`
    $'text {expression} text'
    $"text {expression} text"
    // are all equivalent to:
    `text ${expression} text`

    This is similar to C# but without support for format specifiers.

Incompatibilities

The expression syntax is based on JavaScript's, with a few deliberate incompatibilities in edge cases that I think JavaScript syntax is confusing:

• Identifiers may only have single, interstitial underscores

  • Valid: this_is_totally_valid
  • Invalid: _foo, foo__bar, foo_, $foo

• Exponentiation isn't chainable

  • Should 2**3**2 be (2**3)**2 = 64, or 2**(3**2) = 512? JS and Python both say 512, but I think that's confusing because it's the other way around for other non-associative operators, e.g. 2/3/4 = (2/3)/4. In Mechanical, 2**3**2 is a syntax error
  • (Just like JS, -2**2 is also a syntax error. This actually differs from Python, where -2**2 = -(2**2) = -4, but that's confusing because it look like it could be (-2)**2 = 4)

• Comparisons are chainable

  • In JS, 3 > 2 > 1 is false, which is confusing. In Mechanical, not only does a > b > c behave as expected, but so does a > b >= c == d, or a == b < c == d < e == f. This feature is inspired by Python, however unlike Python, constructions like a < b > c are prohibited (they have to point the same way), and != can't be chained at all.
  • (!= can't be chained because should 1 != 2 != 1 be true or false?)

• No holes in arrays (trailing commas are allowed though)

  // valid:
  [ 1, 2 ]
  [ 1, 2, ]
  [
    1,
    2,
  ]
  
  // invalid:
  [ 1, , 2 ]
  [ 1, 2,, ]

• Mechanical records are much more restricted than JS objects. Field names must be valid identifiers, not arbitrary strings or numerals, and cannot be quoted. They also cannot be computed, cannot be [method definitions], and there are no getters or setters. Trailing commas are allowed though!

  // valid
  { valid_identifier: 1 }
  
  // invalid
  {
    invalid__ident: 1,
    _invalid: 2,
    $invalid: 3,
    "not an identifier at all": 4,
    5: 6,
    [compute(7, 8)]: 9,
    method() {
      Return 10
    },
  }

• Only arrow function expressions (e.g. x => 2*x) are supported (no function (x) { return 2*x }-style function expressions), functions must take at least one argument (all functions are pure, so what would a no-argument function do?), and trailing commas aren't allowed (TODO: method-calling syntax/UFCS, named args when >2 params)

  // valid
  x => 2*x
  (x, y) => sqrt(x**2 + y**2)
  
  // invalid
  () => 1
  (x, y,) => sqrt(x**2 + y**2)
  function (x) { return 2*x }

• No bitwise operators

  • We have none of ~, &, |, <<, >>, >>> built-in, but I hope to introduce a built-in macro

• JS-specific things that don't make sense with Mechanical semantics:

  • No "strict in/equality" ===/!==. Regular in/equality ==/!= is already strict
  • No increment/decrement operators ++/--
  • No in or instanceof relations
  • No assignment operators =, +=, -=, *=, /=, etc
  • No comma operator

License: Blue Oak or MIT

You may use Mechanical under either of our permissive licenses, the highly readable Blue Oak Model License, or the more standard MIT license.