*********************************** Javascript and Node Code Generators *********************************** There are two javascript code generators, ``node`` and ``javascript``. There are two differences between the two: the ``javascript`` code generator when called to output an HTML file will also generate a basic HTML document with the generated code inside a `` As our intention is to develop something that runs in the browser questions naturally arise: - How to interact with HTML elements? - More importantly, when does the generated Idris code start? Starting point of the Idris generated code ------------------------------------------ The generated JavaScript for your program contains an entry point. The ``main`` function is compiled to a JavaScript top-level expression, which will be evaluated during the loading of the ``script`` tag and that is the entry point for Idris generated program starting in the browser. Interaction with HTML elements ------------------------------ As sketched in the Short Example section, the FFI must be used when interaction happens between Idris generated code and the rest of the Browser/JS ecosystem. Information handled by the FFI is separated into two categories. Primitive types in Idris FFI, such as Int, and everything else. The everything else part is accessed via AnyPtr. The ``%foreign`` construction should be used to give implementation on the JavaScript side. And an Idris function declaration to give ``Type`` declaration on the Idris side. The syntax is ``%foreign "browser:lambda:js-lambda-expression"`` . On the Idris side, primitive types and ``PrimIO t`` types should be used as parameters, when defining ``%foreign``. This declaration is a helper function which needs to be called behind the ``primIO`` function. More on this can be found in the FFI chapter. Examples for JavaScript FFI --------------------------- console.log ----------- .. code-block:: idris %foreign "browser:lambda: x => console.log(x)" prim__consoleLog : String -> PrimIO () consoleLog : HasIO io => String -> io () consoleLog x = primIO $ prim__consoleLog x String is a primitive type in Idris and it is represented as a JavaScript String. There is no need for any conversion between the Idris and the JavaScript. setInterval ----------- .. code-block:: idris %foreign "browser:lambda: (a,i)=>setInterval(a,i)" prim__setInterval : PrimIO () -> Int32 -> PrimIO () setInterval : (HasIO io) => IO () -> Int32 -> io () setInterval a i = primIO $ prim__setInterval (toPrim a) i The ``setInterval`` JavaScript function executes the given function in every ``x`` millisecond. We can use Idris generated functions in the callback as far as they have the type ``IO ()`` . HTML Dom elements ----------------- Lets turn our attention to the Dom elements and events. As said above, anything that is not a primitive type should be handled via the ``AnyPtr`` type in the FFI. Anything complex that is returned by a JavaScript function should be captured in an ``AnyPtr`` value. It is advisory to separate the ``AnyPtr`` values into categories. .. code-block:: idris data DomNode = MkNode AnyPtr %foreign "browser:lambda: () => document.body" prim__body : () -> PrimIO AnyPtr body : HasIO io => io DomNode body = map MkNode $ primIO $ prim__body () We create a ``DomNode`` type which holds an ``AnyPtr``. The ``prim__body`` function wraps a lambda function with no parameters. In the Idris function ``body`` we pass an extra ``()`` parameter and the we wrap the result in the ``DomNode`` type using the ``MkNode`` data constructor. Primitive values originated in JavaScript ----------------------------------------- As a countinuation of the previous example, the ``width`` attribute of a DOM element can be retrieved via the FFI. .. code-block:: idris %foreign "browser:lambda: n=>(n.width)" prim__width : AnyPtr -> PrimIO Bits32 width : HasIO io => DomNode -> io Bits32 width (MkNode p) = primIO $ prim__width p Handling JavaScript events -------------------------- .. code-block:: idris data DomEvent = MkEvent AnyPtr %foreign "browser:lambda: (event, callback, node) => node.addEventListener(event, x=>callback(x)())" prim__addEventListener : String -> (AnyPtr -> PrimIO ()) -> AnyPtr -> PrimIO () addEventListener : HasIO io => String -> DomNode -> (DomEvent -> IO ()) -> io () addEventListener event (MkNode n) callback = primIO $ prim__addEventListener event (\ptr => toPrim $ callback $ MkEvent ptr) n In this example shows how to attach an event handler to a particular DOM element. Values of events are also associated with ``AnyPtr`` on the Idris side. To seperate ``DomNode`` form ``DomEvent`` we create two different types. Also it demonstrates how a simple callback function defined in Idris can be used on the JavaScript side. Directives ---------- The javascript code generators accepts three different directives about how compact and obfusacted the generated code should be. The following examples show the code generated for the ``putStr`` function from the prelude for each of the three directives. (``--cg node`` is used in the examples below, but the behavior is the same when generating code to be run in browsers with ``--cg javascript``). With ``idris2 --cg node --directive pretty`` (the default, if no directive is given), a basic pretty printer is used to generate properly indented javascript code. .. code-block:: javascript function Prelude_IO_putStr($0, $1) { return $0.a2(undefined)($7 => Prelude_IO_prim__putStr($1, $7)); } With ``idris2 --cg node --directive compact``, every toplevel function is declared on a single line, and unneeded spaces are removed: .. code-block:: javascript function Prelude_IO_putStr($0,$1){return $0.a2(undefined)($7=>Prelude_IO_prim__putStr($1,$7));} Finally, with ``idris2 --cg node --directive minimal``, toplevel function names are (with a few exceptions like the ones from the static preamble) obfuscated to reduce the size of the generated javascript file: .. code-block:: javascript function $R3a($0,$1){return $0.a2(undefined)($7=>$R3b($1,$7));}