在GHCI,我类型如下:
map (+1) [1..10]
which returns [2,3,4,5,6,7,8,9,10,11]
迄今情况良好。
现为第一类:
min (map (+1) [1..10])
我收到以下错误信息:
No instance for (Show ([b0] -> [b0]))
arising from a use of `print
Possible fix: add an instance declaration for (Show ([b0] -> [b0]))
In a stmt of an interactive GHCi command: print it
这对我来说非常奇怪。 为什么我会认为我是谁试图印刷任何论点? 我如何确定这一点?
问题是<代码>min。 有两个论点(至少是两个论点),但你只提出具体论点;你希望<条码>最低,该文本在名单上运作。
Specifically, this error occurs because GHCi tries to print out the result of the expressions you evaluate, but in this case, you ve applied min to one argument, producing another function.1 GHCi dutifully tries to print this out, but it fails, because functions are not an instance of Show, the standard type-class for things that can be represented as strings for display.
1 In Haskell, all functions take one argument, and functions of multiple arguments are built up from functions returning other functions; for instance, a function adding two integers might have the type Integer -> Integer -> Integer, which is Integer -> (Integer -> Integer) — a function taking an Integer and returning another function which itself takes an Integer, and returns an Integer. Thankfully, you don t have to think about this in such explicit terms very often, or it d get very confusing!
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