（Data.Functor.Classes.Show1 ExprF）没有实例(No instance for (Data.Functor.Classes.Show1 ExprF))
我写了一个小程序来找到一个数字的素数因子分解。 除了main函数之外，所有东西似乎都在编译， main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 1I've written a small program to find the prime factorization of a number. Everything seems to compile except for the main function, which complains about not being able to find a Show1 instance.
{-# 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 10Logs:
% 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最满意答案
Show for Fix的实例是： Show1 f => Show (Fix f) ，这就是编译器期望Show1 ExprF 。
Show1可以在Data.Functor.Classes下的Data.Functor.Classes找到，并且有一个TH脚本可以从Text.Show.Deriving -compat中的Text.Show.Deriving派生它。
The instance of Show for Fix is: Show1 f => Show (Fix f), which is why the compiler expects Show1 ExprF.
Show1 can be found in base under Data.Functor.Classes, and there is a TH script to derive it in Text.Show.Deriving from deriving-compat.
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