initial

.gitignore

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+.mypy_cache
+gen/

Justfile

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+watch:
+	watchexec -ce py -i gen 'mypy *.py && python gen.py && mypy gen/*.py'

arith.ag

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+iface Node {
+	val: int,
+}
+
+node Expr : Node {
+	<l:Expr> "+" <r:Expr> => {
+		self.val = l.val + r.val;
+	}
+	<l:Expr> "*" <r:Expr> => {
+		self.val = l.val * r.val;
+	}
+	<n:r"[0-9]+"> => { self.val = parseInt(n); }
+}

gen.py

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+import textwrap
+import os
+from lark import Lark
+
+p = Lark(open("grammar.lark").read())
+
+if __name__ == "__main__":
+	with open("arith.ag") as f:
+		data = f.read()
+
+	t = p.parse(data)
+	print(t)
+
+	if not os.path.exists("gen"):
+		os.makedirs("gen")
+	with open("gen/arith.py", "w") as f:
+		f.write(textwrap.dedent("""
+			from typing import Generic, TypeVar
+			T = TypeVar('T')
+			class Thunk(Generic[T]):
+				pass
+		"""))

grammar.lark

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+start: decl*
+
+decl: iface
+	| node
+
+iface: IFACE IDENT "{" iface_field (COMMA iface_field)? COMMA? "}"
+iface_field: IDENT COLON IDENT
+
+node: NODE IDENT COLON IDENT "{" node_prod* "}"
+node_prod: prod THICCARROW "{" (equation SEMI)* "}"
+prod: sym*
+sym: IDENT
+	| LANG IDENT COLON node_ref RANG
+	| STRING
+node_ref: IDENT
+	| STRING
+equation: expr EQ expr
+
+expr: expr DOT expr
+	| expr ADD expr
+	| expr MUL expr
+	| expr LPAR args RPAR
+	| IDENT
+args: expr (COMMA expr)? COMMA?
+
+IDENT: /([a-zA-Z][a-zA-Z0-9_]*)|(_[a-zA-Z0-9_]+)/
+IFACE: "iface"
+NODE: "node"
+LANG: "<"
+RANG: ">"
+ADD: "+"
+MUL: "*"
+COMMA: ","
+COLON: ":"
+LPAR: "("
+RPAR: ")"
+EQ: "="
+DOT: "."
+SEMI: ";"
+THICCARROW: "=>"
+
+%import python.STRING
+%import common.WS
+%ignore WS

main.py

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+from typing import Generic, TypeVar, Optional, Callable
+
+T = TypeVar("T")
+
+class Thunk(Generic[T]):
+	value: Optional[T]
+
+	def __init__(self, func: Callable[[], T]):
+		self.func = func
+		self.value = None
+
+	def get(self) -> T:
+		if self.value is None:
+			self.value = self.func()
+		return self.value
+
+class Node:
+	value: Thunk[int]
+
+class Add(Node):
+	def __init__(self, left: Node, right: Node):
+		self.value = Thunk(lambda: left.value.get() + right.value.get())
+
+class Mul(Node):
+	def __init__(self, left: Node, right: Node):
+		self.value = Thunk(lambda: left.value.get() * right.value.get())
+
+class Lit(Node):
+	def __init__(self, num: int):
+		self.num = num
+		self.value = Thunk(lambda: num)
+
+if __name__ == "__main__":
+	tree = Add(Mul(Lit(3), Lit(4)), Lit(5))
+	print(tree.value.get())

mypy.ini

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+[mypy]
+strict = True