没有实例(Data.Functor.Classes.Show1 ExprF)

Question

我编写了一个小程序来寻找一个数字的素因式分解。除了main函数之外，所有东西似乎都是编译的，该函数抱怨无法找到Show1实例。

{-# LANGUAGE DeriveFunctor #-}

module FactorAnamorphism where

import Data.Functor.Foldable
import Data.List

nextPrimeFactor :: Integer -> Maybe Integer
nextPrimeFactor n = find (\x -> n `mod` x /= 0) [2..(floor $ sqrt $ fromIntegral n)]

data ExprF r = FactorF Integer | MultF r r deriving (Show, Functor)
type Expr = Fix ExprF

factor :: Integer -> Expr
factor = ana coAlg where
  coAlg fac = case (nextPrimeFactor fac) of
    Just prime -> MultF prime (fac `div` prime)
    Nothing    -> FactorF fac

main :: IO ()
main = putStrLn $ show $ factor 10

日志：

 % stack build
haskell-playground-0.1.0.0: build (lib + exe)
Preprocessing library haskell-playground-0.1.0.0...
Preprocessing executable 'factor-anamorphism' for
haskell-playground-0.1.0.0...
[1 of 1] Compiling FactorAnamorphism ( app/FactorAnamorphism.hs, .stack-work/dist/x86_64-osx/Cabal-1.24.2.0/build/factor-anamorphism/factor-anamorphism-tmp/FactorAnamorphism.o )

/Users/ian/proj/macalinao/haskell-playground/app/FactorAnamorphism.hs:22:19: error:
    • No instance for (Data.Functor.Classes.Show1 ExprF)
        arising from a use of ‘show’
    • In the second argument of ‘($)’, namely ‘show $ factor 10’
      In the expression: putStrLn $ show $ factor 10
      In an equation for ‘main’: main = putStrLn $ show $ factor 10

--  While building package haskell-playground-0.1.0.0 using:
      /Users/ian/.stack/setup-exe-cache/x86_64-osx/Cabal-simple_mPHDZzAJ_1.24.2.0_ghc-8.0.2 --builddir=.stack-work/dist/x86_64-osx/Cabal-1.24.2.0 build lib:haskell-playground exe:factor-anamorphism exe:haskell-playground-exe --ghc-options " -ddump-hi -ddump-to-file"
    Process exited with code: ExitFailure 1

Answer 1

Show for Fix的实例是：Show1 f => Show (Fix f)，这就是编译器期望Show1 ExprF的原因。

Show1可以在Data.Functor.Classes下的base中找到，并且在Text.Show.Deriving中有一个从派生-compat派生它的脚本。