move ast definitions into its own package

source/ast/ast.hon

@@ -1 +1,50 @@
-# todo: move ast stuff from parser module to here
+import "@source/lexer"
+
+hide TokenId :: alias lexer.TokenId
+
+NodeId :: distinct u32
+ExtraId :: distinct u32
+
+NO_ID_NODE :: maxval!(NodeId)
+NO_ID_EXTRA :: maxval!(ExtraId)
+
+# unsafe access on its own; NodeKind serves as a the tag
+# that determines the variant.
+NodeData :: union {
+    token_id TokenId
+    node_id NodeId
+    extra_id ExtraId
+}
+
+Node :: struct {
+    kind NodeKind
+    main_token TokenId
+
+    data0 NodeData = NodeData{ node_id = NO_ID_NODE }
+    data1 NodeData = NodeData{ node_id = NO_ID_NODE }
+}
+
+NodeKind :: enum {
+    # LITERALS:
+    # * main_token: token literal
+    literal_int
+    literal_float
+    literal_string
+
+    # IDENTIFIER EXPR:
+    # * main_token: token literal
+    expr_identifier
+
+    expr_unary
+    expr_binary
+
+    # DECL:
+    # * main_token: symbol name (identifier)
+    # * data0: ExtraId
+    #   + extra[data0]: NodeId - type identifier node
+    #   + extra[data0 + 1]: NodeId - initializer expression
+    # * data1: TokenId of either `::` or `=` indicating mutability
+    stmt_decl
+
+    invalid
+}