[英]haskell cannot construct the infinite type
我認為以下代碼應該工作:
sum_up x = loop_it 0 x
where loop_it sum i | i > 0 = loop_it sum+i i-1
| i == 0 = sum
但是我收到了這個錯誤:
<interactive>:3:15: error:
• Occurs check: cannot construct the infinite type:
t2 ~ (t0 -> t2) -> t2
Expected type: t2 -> t2
Actual type: t2 -> (t0 -> t2) -> t2
• In an equation for ‘sum_up’:
sum_up x
= loop_it 0 x
where
loop_it sum i
| i > 0 = loop_it sum + i i - 1
| i == 0 = sum
• Relevant bindings include
loop_it :: t2 -> t2 (bound at <interactive>:3:15)
為什么不編譯?
你需要圍繞遞歸調用loop_it
的參數括起來:
sum_up x = loop_it 0 x
where loop_it sum i | i > 0 = loop_it (sum+i) (i-1) -- <- Here
| i == 0 = sum
如果你不像那樣對它進行分組,編譯器會隱式地將它分組如下:
((loop_it sum)+(i i))-1
...這可能不是你想要的,因為這意味着:“將loop_it
應用於sum
,然后將其添加到ii
(即i
應用於自身),然后減去1。
這是因為函數應用程序在Haskell中具有最高優先級,因此函數應用程序比算術綁定更緊密。
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