如何在haskell中使用do和while循环

Question

我尝试使用加法并打印每个步骤，但它不起作用，有人可以帮帮我吗？

addition x = x+1
acc_addition xs = do print xs
                     let result = addition xs
                         if result == 5
                            then return ()
                            else do
                                 print result
                                 addition result

Answer 1

您已经很接近了，您只需在最后一步调用acc_addition 而不是addition。从语法上讲，您还需要为您的 let 语句添加一个 in：

addition x = x+1
acc_addition xs = do print xs
                     let result = addition xs in
                         if result == 5
                            then return ()
                            else do
                                 print result
                                 acc_addition result

当通过ghci运行时：

*Main> acc_addition 1
1
2
2
3
3
4
4

它打印两次的原因当然是你有两个打印语句。