import Test.QuickCheck
import System.Random
data Letter = G | B deriving(Show, Eq, Bounded)
arbitraryLetter :: Gen Letter
arbitraryLetter = choose (G,B)
我正在编译并获取此错误
⢠No instance for (Random Letter) arising from a useof âchooseâ
⢠In the expression: choose (G, B)
In an equation for âarbitraryLetterâ:
arbitraryLetter = choose (G, B)