User-defined scopes

Introduction

Before we dive into how to define your own scopes, we need to define some language for talking about scopes, and identify some components of scopes. We will start with scopes that are common in most programming languages: if and while.

Scopes can consist of one or more blocks of code. For while loops, most languages do not give a name to the block, but the block is chunk of code that is executed on each loop iteration. In the Icarus standard library, we would write while (condition) do { ... }, so the block would be do.

If statements in most languages can have either one or two blocks: The then block is mandatory, and the else block is optional.

You may be wondering about “else if”, but this is not actually a special construct in all languages. In particular in Icarus, we allow “else if”, but it is syntactic sugar. That is,

if (a) then {
  ...
} else if (b) then {
  ...
} else {
  ...
}

is syntactic sugar for

if (a) then {
} else {
  if (b) then {
    ...
  } else {
    ...
  }
}

A simple example

Let’s start by defining a relatively simple scope named forever which will loop forever. Because each scope needs at least one block, and blocks must be named in Icarus, we will choose the name do for the block.

The implementation of this scope looks like:

forever ::= scope {
  enter ::= jump () { goto do() }

  do ::= block {
    before ::= () -> () {}
    after ::= jump () { goto do() }
  }
}

// Usage
forever () do {
  do_something()
}

When a call to the forever scope is reached, the enter jump will be executed. The jump says that it should go to the do block, so control jumps to the before function in the do block. Once the function executes (it is trivial in this case, but you may imagine wanting to do something interesting here), the body of the do block is executed. In this example, that’s the call to do_something. When the body of the do block completes, the do block’s after jump is executed which once again directs control back to the do block itself, restarting the loop.

TODO: Dive into more details here.