Category

Category is a way to limit the place of calling a specified block or sentence.

###Category on Sentences Some sentences are only make sense to call in some blocks, like break and repeat. Multiple blocks can share a same category, and a sentence is allowed to be configured to be appear inside multiple categories of blocks.

For example, if we want to limit that break can only be used inside repeat:

category
	start REPEAT
	closable
block repeat ...

category
	inside REPEAT
sentence break ...

A block can only have one start, but a sentence can have multiple inside. closable means that this block can be ended by end.

Because of the category configuration, the following statements are correct:

repeat
	break
end

repeat
	if true
		break
	end
end

Although if is not in category REPEAT, but repeat is. So break can be used in the if in repeat.

And this is not correct because break does not appear inside repeat or any other blocks that in caregory REPEAT:

try
	break
finally
end

inside is not allowed on blocks, it is the only allowed configuration for sentences.

###Category on Blocks and Block Chains You can specify follow on blocks the build a block chain!

For example, you can use block to build the if statement:

block (sentence body) if (condition)
	select condition
		case true
			body
	end
end
``

But what about `else if` and `else`?

To enable that, Tinymoe provide a feature called **block chain**. If you declare `if`, `else if` and `else` in the following way:

category start IFELSE if closable block (sentence body) if (condition) ...

category (signal) follow IFELSE if start IFELSE if closable block (sentence body) else if (expression condition) ...

category (signal) follow IFELSE if closable block (sentence body) else ...

Then `else if` and `else` can be used right after `if` because of the `follow` configuration. `else if` and `else` cannot be used without `if` also because of the `follow` configuration. `if`, `else if` and `else` can be the last block in a block chain because of the `closable` configuration.

The `follow` configuration require a block should be appear right after other blocks in the specified category. Multiple `follow` are allowed on a block.

If a block has a `follow` configuration, than `category (signal)` is required because a block should receive the return value from the previous block. For example:

if condition one A else if condition two B else if condition three C else D end

All four blocks, `if`, two `else if` and `else`, **are executed sequencially**. When `condition one` success, `if` should pass the imformation to the next `else if` to tell it to skip, and `else if` will tell the second `else if` to skip, and it will tell the last `else` to skip.

A block can pass information to the next block by writing `the result` variable. A block can receive information from the previous block by `signal` variable specified in `category (signal)`.

If one of the previous block succeed, then the current `else if` should not evaluate both the condition and the variable. This is why else if uses `(expression condition)`.

More explanation for `if` series blocks can be found in [Standard Library: If Else](https://github.com/vczh/tinymoe/wiki/Standard-Library-If-Else).