修复带有可区分联合的直接循环引用

Question

我正在制作一个迷你 Pascal 解析器，this Pascal syntax 支持声明无限类型：

program ::= program identifier ; block .

block ::= variable-declaration-part procedure-declaration-part statement-part

procedure-declaration-part ::= { procedure-declaration ; }

procedure-declaration ::= procedure identifier ; block

我尝试为此语法创建 AST 域类型：

type Program = Identifier * Block
and Block = VariableDeclaration list option * ProcedureDeclaration list option * Statement list
and ProcedureDeclaration = Identifier * Block

但是编译器会抛出错误，因为我使用了类型别名，并且它们在编译时被删除了：

This type definition involves an immediate cyclic reference through an abbreviation

有人说我可以用包装的可区分联合类型解决这个问题，我该怎么办？

Answer 1

and ProcedureDeclaration = ProcedureDeclaration of Identifier * Block

现在它不仅仅是一个元组，而是一个单一大小写的 DU 类型，您可以使用例如

创建它

ProcedureDeclaration(someID, someBlock)

并分解为

match somePD with
| ProcedureDeclaration(i,b) -> ...