是否可以生成任意字符串，并避免在QuickCheck中重复指定？

Question

给定

data MyType = MyType ...

makeMyType :: String -> String -> String -> MyType
-- ...

type StringThing = String

makeMyType期望的字符串分别在哪里：

• -一些自定义字符串的分隔字符串（例如， "Hilary-Jeb-David-Charles" ），
• 一串4个大写字母，以及
• 一个. 介于1到26之间的整数的定界字符串，每个字符串都填充有零到两个字符（例如"04.23.11.09" ）

我可以使用QuickCheck生成类似以下内容的足够的测试用例

{-# LANGUAGE TypeSynonymInstances, FlexibleInstances #-}
import Test.QuickCheck
import Data.List (intercalate)
import Text.Printf (printf)

-- This really should be an arbitrary string of random length in some range
instance Arbitrary StringThing where
        arbitrary = elements ["FUSHFJSHF","KLSJDHFLSKJDHFLSKJDFHLSKJDHFLSKJOIWURURW","GHSHDHUUUHHHA"]

instance Arbitrary MyType where
        arbitrary = do
                -- This repetition feels unnecessary
                w1 <- elements ['A'..'Z']
                w2 <- elements ['A'..'Z']
                w3 <- elements ['A'..'Z']
                w4 <- elements ['A'..'Z']
                c1 <- elements someListOfCustomStuff
                c2 <- elements someListOfCustomStuff
                c3 <- elements someListOfCustomStuff
                c4 <- elements someListOfCustomStuff
                r1 <- choose (1,26)
                r2 <- choose (1,26)
                r3 <- choose (1,26)
                r4 <- choose (1,26)
                return $ makeMyType (intercalate "-" [c4,c3,c2,c1])
                                    [w1,w2,w3,w4]
                                    (intercalate "." $ (printf "%02d") <$> ([r1,r2,r3,r4] :: [Int]))

prop_SomeProp :: MyType -> StringThing -> Bool
prop_SomeProp mt st = ...

但StringThing确实应该采取一些范围内随机长度，和同规格的重复的大写字母的任意字符串的所有w... S， c.. s和r....小号似乎没有必要。

QuickCheck中有没有一种方法可以：

1. 生成长度在一定范围内，限制在某些字符内的随机字符串，并且
2. 使用elements “共享”规范或在多个值中choose ？

Answer 1

是。 Haskell擅长将问题分解出来！ 您当然可以命名和共享子表达式，例如elements ['A'..'Z']

capitals = elements ['A'..'Z']

instance Arbitrary StringThing where
        arbitrary = do
          l <- choose (1,50) -- this is your string length
          replicateM l capitals

对于MyType，还可以大量使用copyM：

instance Arbitrary MyType where
        arbitrary = do
                ws <- replicateM 4 capitals
                cs <- replicateM 4 (elements someListOfCustomStuff)
                rs <- replicateM 4 (choose (1,26))
                return $ makeMyType (intercalate "-" cs)
                                    ws
                                    (intercalate "." $ (printf "%02d") <$> (rs :: [Int]))