Anselme v2.0.0-alpha rewrite

Woke up and felt like changing a couple things. It's actually been worked on for a while, little at a time...

The goal was to make the language and implementation much simpler. Well I don't know if it really ended up being simpler but it sure is more robust.

Main changes:
* proper first class functions and closures supports! proper scoping rules! no more namespace shenanigans!
* everything is an expression, no more statements! make the implementation both simpler and more complex, but it's much more consistent now! the syntax has massively changed as a result though.
* much more organized and easy to modify codebase: one file for each AST node, no more random fields or behavior set by some random node exceptionally, everything should now follow the same API defined in ast.abstract.Node

Every foundational feature should be implemented right now. The vast majority of things that were possible in v2 are possible now; some things aren't, but that's usually because v2 is a bit more sane.
The main missing things before a proper release are tests and documentation. There's a few other things that might be implemented later, see the ideas.md file.

parser/expression/to_ast.lua

@@ -0,0 +1,50 @@
+--- transform an expression string into raw AST
+
+local primary, secondary
+
+local comment = require("parser.expression.comment")
+
+-- parse an expression, starting from a secondary element operating on operating_on_primary
+-- returns expr, remaining
+local function from_secondary(source, s, limit_pattern, current_priority, operating_on_primary)
+	s = source:consume(s:match("^(%s*)(.*)$"))
+	current_priority = current_priority or 0
+	-- if there is a comment, restart the parsing after the comment ends
+	local c, c_rem = comment:search(source, s, limit_pattern)
+	if c then return from_secondary(source, c_rem, limit_pattern, current_priority, operating_on_primary) end
+	-- secondary elements
+	local exp, rem = secondary:search(source, s, limit_pattern, current_priority, operating_on_primary)
+	if exp then return from_secondary(source, rem, limit_pattern, current_priority, exp) end
+	-- nothing to apply on primary
+	return operating_on_primary, s
+end
+
+--- parse an expression
+-- current_priority: only elements of strictly higher priority will be parser
+-- limit_pattern: set to a string pattern that will trigger the end of elements that would otherwise consume everything until end-of-line (pattern is not consumed)
+-- fallback_exp: if no primary expression can be found, will return this instead. Used to avoid raising an error where an empty or comment-only expression is allowed.
+-- return expr, remaining
+local function expression_to_ast(source, s, limit_pattern, current_priority, fallback_exp)
+	s = source:consume(s:match("^(%s*)(.*)$"))
+	current_priority = current_priority or 0
+	-- if there is a comment, restart the parsing after the comment ends
+	local c, c_rem = comment:search(source, s, limit_pattern)
+	if c then return expression_to_ast(source, c_rem, limit_pattern, current_priority, fallback_exp) end
+	-- primary elements
+	local exp, rem = primary:search(source, s, limit_pattern)
+	if exp then return from_secondary(source, rem, limit_pattern, current_priority, exp) end
+	-- no valid primary expression
+	if fallback_exp then return fallback_exp, s end
+	error(("no valid expression before %q"):format(s), 0)
+end
+
+package.loaded[...] = expression_to_ast
+
+primary = require("parser.expression.primary")
+secondary = require("parser.expression.secondary")
+
+-- return expr, remaining
+return function(source, s, limit_pattern, current_priority, operating_on_primary, fallback_exp)
+	if operating_on_primary then return from_secondary(source, s, limit_pattern, current_priority, operating_on_primary)
+	else return expression_to_ast(source, s, limit_pattern, current_priority, fallback_exp) end
+end