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<h1>Modal in N Minutes</h1>
<pre style="white-space: pre-wrap;">
Modal is a programming language that operates on trees, represented as strings.
(ref: x) -> comment specific to the <a href="https://git.sr.ht/~rabbits/modal">reference implementation</a>
The leaf in a tree is a symbol.
A symbol is a string, with some limitations.
A symbol cannot be equal to '<>'.
A symbol cannot contain the characters ')', ' ', and '('.
These are are all special non-symbol elements, to be described.
Here are some symbols:
foo
12345
.'.'.'
The character set allowed in symbols is undefined. (ref: ASCII minus control characters)
Children in the tree are defined by sequences.
A sequence is a collection of trees, separated by spaces.
Trees' children may also be referred to as 'elements', which are also trees (sub-trees)
A sequence is itself a tree.
Here are some trees:
a -> a tree consisting of a symbol
(a b) -> a tree consisting of a sequence of two symbols
(a (b)) -> a tree consistent of a sequence of 'a' and the tree '(b)' (a sequence with one element, a symbol)
Definitions are string replacement rules created by <>.
<> is a special string that cannot be a symbol
(ie, cannot be the leaf of a tree)
A definition is a string replacement operation.
A definition takes two "tokens", which are replacement operations that act upon trees.
(Traversal order is undefined (ref: breadth-first))
A definition can have up to two replacement operations.
If a definition only has one operation, the right side is "".
A definition says:
"Take a tree, that, when represented as a string, matches to
the rule [left], and replace it with [right]"
<> (a) (b) says: define a rule that replaces (a) with (b).
Replacements may contain registers, which are preceded by ?
(ref: and are a single lowercase letter)
e.g. ?a is a register
A register says:
"find the element in this position in the input tree, store it in
this register, then place the element in the output tree in its
associated position"
Some definitions:
<> a b
<> foo
<> (swap ?x ?y) (?y ?x)
<> (if (#t) ?b) (?b)
There are three special registers, ?:, ?~, ?* and the lambda register.
?: is the output register
Instead of replacing an element, when the definition is
applied, it will consume the element and print it to the
console.
?~ is the input register
instead of replacing an element, when the definition is applied, it
will read an element (ref: whitespace-separated)_ from the console.
?* TODO
?(BODY) is the lambda register, where BODY are the two
tokens in a definition.
Rules created that way exist only for the length of one rewrite
and must match what is found immediately after.
Execution of a modal program consists of:
1. Parsing all definitions in order, leaving only a
tree and its elements.
2. Subsequently applying rules to this tree until no
more rules apply.
</pre>
For more information, see <a href="https://wiki.xxiivv.com/site/modal">Modal</a>.
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