[英]Can some explain the reflection package API in *very* simple terms?
I'm having a hard time understanding the documentation/examples out there describing the reflection package.我很难理解描述反射包的文档/示例。 I'm an imperative-programming veteran but a Haskell newb.
我是命令式编程的老手,但还是 Haskell 新手。 Can you walk me through a very simple introduction?
你能告诉我一个非常简单的介绍吗?
Package: https://hackage.haskell.org/package/reflection包: https : //hackage.haskell.org/package/reflection
Edit: to whoever closed this question: this is meant to be a beginner introduction to Haskell reflection.编辑:致关闭这个问题的人:这是对 Haskell 反射的初学者介绍。 The answer below is excellent and others would be useful too, so please reopen.
下面的答案很好,其他的也很有用,所以请重新打开。
In the simplest use-case, if you have some configuration information you'd like to make generally available across a set of functions:在最简单的用例中,如果您有一些配置信息,您希望在一组功能中普遍可用:
data Config = Config { w :: Int, s :: String }
you can add a Given Config
constraint to the functions that need access to the configuration:您可以向需要访问配置的函数添加
Given Config
约束:
timesW :: (Given Config) => Int -> Int
and then use the value given
to refer to the current configuration (and so w given
or s given
to refer to its fields):然后使用
given
的值来指代当前配置(因此w given
或s given
指的是其字段):
timesW x = w given * x
With a few other functions, some that use the configuration and some that don't:还有一些其他功能,有些使用配置,有些不使用:
copies :: Int -> String -> String
copies n str = concat (replicate n str)
foo :: (Given Config) => Int -> String
foo n = copies (timesW n) (s given)
you can then run a computation under different configurations that you give
:然后,您可以在您
give
不同配置下运行计算:
main = do
print $ give (Config 5 "x") $ foo 3
print $ give (Config 2 "no") $ foo 4
This is similar to:这类似于:
defining given :: Config
globally, except you can run the computation under multiple configurations in the same program;全局定义
given :: Config
,除非您可以在同一程序中的多个配置下运行计算;
passing the configuration around as an extra parameter to every function, except you avoid the bother of explicitly accepting the configuration and passing it on, like:将配置作为额外参数传递给每个函数,除非您避免显式接受配置并将其传递,例如:
timesW cfg x = w cfg * x foo cfg n = copies (timesW cfg n) (s cfg)
using a Reader
monad, but you don't have to lift everything to an awkward monad- or applicative-level syntax, like:使用
Reader
monad,但您不必将所有内容都提升为笨拙的 monad 或应用程序级别的语法,例如:
timesW x = (*) <$> asks w <*> pure x foo n = copies <$> timesW n <*> asks s
The full example:完整示例:
{-# LANGUAGE FlexibleContexts #-}
import Data.Reflection
data Config = Config { w :: Int, s :: String }
timesW :: (Given Config) => Int -> Int
timesW x = w given * x
copies :: Int -> String -> String
copies n str = concat (replicate n str)
foo :: (Given Config) => Int -> String
foo n = copies (timesW n) (s given)
main = do
print $ give (Config 5 "x") $ foo 3
print $ give (Config 2 "no") $ foo 4
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