我可以命名函数签名吗？
我正在传递一个部分应用的函数。完整的签名是：
import Data.Map as Map
-- Update the correct bin of the histogram based on the min value, bin width,
-- the histogram stored as a map, and the actual value we are interested in.
updateHist :: Double -> Double -> Map.Map Bin Double -> Double ->
Map.Map Bin Double
该函数更新存储直方图数据的 Map。前两个参数给出了我们感兴趣的数据的下限，接下来是直方图的箱宽度。我在程序启动时填写这些值，并将部分应用的函数传递到整个模块。这意味着我有大量具有如下签名的函数：
-- Extra the data out of the string and update the histogram (in the Map) with it.
doSomething :: String -> (Map.Map Bin Double -> Double -> Map.Map Bin Double) ->
Map.Map Bin Double
这一切都很好，但编写“(Map.Map Bin Double -> Double -> Map.Map Bin Double)”相当冗长。我想将它们全部替换为“UpdateHistFunc”作为类型，但由于某种原因我一直失败。
我尝试过：
newtype UpdateHistFunc = Map.Map Bin Double -> Double -> Map.Map Bin Double
失败并出现以下错误：
HistogramForColumn.hs:84:44：输入“->”时解析错误
我做错了什么？
I'm passing around a partially applied function. The full signature is:
import Data.Map as Map
-- Update the correct bin of the histogram based on the min value, bin width,
-- the histogram stored as a map, and the actual value we are interested in.
updateHist :: Double -> Double -> Map.Map Bin Double -> Double ->
Map.Map Bin Double
The function updates a Map which stores data for a histogram. The first two parameters give the bottom bounds of data we are interested, the next is the bin width for the histogram. I fill these values in when the program starts up and pass the partially applied function all over the module. This means I have a ton of functions with a signature like:
-- Extra the data out of the string and update the histogram (in the Map) with it.
doSomething :: String -> (Map.Map Bin Double -> Double -> Map.Map Bin Double) ->
Map.Map Bin Double
This is all fine and dandy, but writing "(Map.Map Bin Double -> Double -> Map.Map Bin Double)" is rather verbose. I'd like to replace them all with "UpdateHistFunc" as a type but for some reason I keep failing.
I tried:
newtype UpdateHistFunc = Map.Map Bin Double -> Double -> Map.Map Bin Double
This failed with the error:
HistogramForColumn.hs:84:44: parse error on input `->'
What am I doing wrong?
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您是否在这里混淆了
type和newtype？使用
type定义一个类型同义词，这似乎是您想要做的，而newtype创建一个需要构造函数名称的新类型，就像使用data。换句话说，您可能想要这样：
...或者可能这样：
后者显然需要“展开”才能应用该功能。
供参考：
data定义了一种新的代数数据类型，它可以是递归的，具有不同的类型类实例，引入了可能的惰性的额外层，等等。newtype定义了一个数据类型，它带有一个带有单个参数的构造函数，它可以是递归的并且具有不同的实例，但仅用于类型检查；编译后，它相当于它包含的类型。type定义一个类型同义词，它不能递归或具有不同的实例，在类型检查时完全展开，相当于一个宏。如果您想了解
data和newtype之间在“额外惰性”方面的语义区别，请比较这两种类型以及它们可能具有的值：例如，您认为这些函数如果分别应用于
undefined与DCon undefined和NCon undefined会做什么？Are you confusing
typeandnewtypehere?Using
typedefines a type synonym, which is what you seem to be trying to do, whereasnewtypecreates a new type that needs a constructor name, like withdata.In other words, you probably want this:
...or maybe this:
The latter obviously needs to be "unwrapped" in order to apply the function.
For reference:
datadefines a new algebraic data type, which can be recursive, have distinct instances of type classes, introduces an extra layer of possible laziness, all that stuff.newtypedefines a data type with a single constructor taking a single argument, which can be recursive and have distinct instances, but only for type checking; after compilation, it's equivalent to the type it contains.typedefines a type synonym, which can't be recursive or have distinct instances, is fully expanded when type checking, and amounts to little more than a macro.If you're wondering about the semantic distinction between
dataandnewtypewhere "extra laziness" is concerned, compare these two types and the possible values they can have:For instance, what do you think these functions will do if applied to
undefinedvs.DCon undefinedandNCon undefined, respectively?