我编写了一个小程序来查找数字的素因式分解。除了 main 函数(它提示无法找到 Show1 实例)外，所有东西似乎都能编译。
{-# 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
最佳答案
Show的Fix实例为:Show1 f => Show (Fix f)，这就是为什么编译器期望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TH脚本可以从deriving-compat派生它。
关于haskell - 没有(Data.Functor.Classes.Show1 ExprF)的实例，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/43567005/