Intermediate Directions and Movement

Relative directions rotate along with their pattern. This:

[ > Player | Crate ] -> [ > Player | > Crate ]

gets compiled to the following set of rules:

UP [ UP player | crate ] -> [ UP player | UP crate ] 
DOWN [ DOWN player | crate ] -> [ DOWN player | DOWN crate ] 
LEFT [ LEFT player | crate ] -> [ LEFT player | LEFT crate ] 
RIGHT [ RIGHT player | crate ] -> [ RIGHT player | RIGHT crate ]

You can restrict patterns so that they're only matched in a particular direction (by default, when the interpreter tries to find places to apply to rule, it tries all four rotations of it). To implement gravity, we could do

DOWN [ stationary Object ] -> [ > Object ]

Which compiles to

DOWN [ stationary Object ] -> [ DOWN Object ]

It would probably have been simpler to write the rule as

[ stationary Object ] -> [ DOWN Object ]

which is the same thing.

Another good illustration of why rule direction might be important would be if you're making a word game.

[ C | A | T ] -> WIN

would look for words in all four directions, so TAC would also match this rule. To fix it, you would restrict the rule to read left-to-right as follows:

 Editright [ C | A | T ] -> WIN

Huzzah!

There are horizontal directons as well - if you want something that moves horizontally when you move horizontally, but ignores you otherwise, do this:

[ Horizontal  Player ] [ Crate ] -> [  Horizontal  Player ] [  Horizontal Crate  ]

and get this:

(Vertical is also a keyword that you can use).

Vertical and horizontal are both keywords, but is there a way of reducing the following to a single instruction?

[ ^ Player | Crate ] -> [ ^ Player | ^ Crate ]
[ v Player | Crate ] -> [ v Player | v Crate ]

There is, and it's this:

[ Perpendicular Player | Crate ] -> [ Perpendicular Player | Perpendicular Crate ]

Parallel is also a keyword, and means what you think it might. 😀

Tip: if you don't know what something does in the examples (or in your own code 😆), it can be helpful to add the debug flag to the prelude to see how the compiler interprets it. Or you can just click on the debug button in the editor console.