当我尝试从http://michaeldadams.org/papers/scrap_your_zippers/ScrapYourZippers.hs编译代码时,我得到:
ScrapYourZipper.hs:249:15: Not in scope: type variable ‘hole’
ScrapYourZipper.hs:251:27: Not in scope: type variable ‘root’
ScrapYourZipper.hs:252:20: Not in scope: type variable ‘hole’
ScrapYourZipper.hs:252:25: Not in scope: type variable ‘root’
发生这种情况的代码部分:
245 data Context hole root where
246 CtxtNull :: Context a a
247 CtxtCons ::
248 forall rights parent. (Data parent) =>
249 Left (hole -> rights)
250 -> Right rights parent
251 -> Context parent root
252 -> Context hole root
有什么想法/指示出了什么问题?
PS:抱歉帖子命名不好,想不出有意义的东西。