小编jcr*_*vao的帖子

是否可以为"非monad"定义实例约束,以便定义两个非重叠实例,一个用于monadic值,另一个用于非monadic值？

一个简化的例子:

{-# LANGUAGE MultiParamTypeClasses  #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances      #-}
{-# LANGUAGE FlexibleContexts       #-}
{-# LANGUAGE OverlappingInstances   #-}

class WhatIs a b | a -> b where
  whatIs :: a -> b

instance (Show a) => WhatIs a String where
  whatIs = show

instance (Monad m, Functor m, Show a) => WhatIs (m a) (m String) where
  whatIs x = fmap show x


main :: IO ()
main = do
  let x = 1 :: Int …

我试图定义这个函数来重新组合三个对列表:

{-# LANGUAGE RankNTypes #-}

mapAndZip3 :: (forall x. x -> f x) -> [a] -> [b] -> [c] 
                                   -> [(f a, f b, f c)]
mapAndZip3 f la lb lc = zipWith3 (\a b c -> (f a, f b, f c)) la lb lc


main = do
    let x = mapAndZip3 (fst) [(1,"fruit"), (2,"martini")] 
                             [("chips","fish"),("rice","steak")]
                             [(5,"cake"),(4,"pudding")]
    print x -- was expecting [(1,"chips",5), (2,"rice",4)]

起初我没有包括RankNTypes或者forall,但是在看到这个之后,即liftTup定义,我认为它应该足够了.

但很明显,事实并非如此,因为我仍然会收到错误:

mapAndZip3.hs:8:25:
Couldn't match type `x' …

我正在尝试定义Markdown newtype,并使用GeneralizedNewtypeDeriving自动定义新实例:

import Text.Markdown
import Yesod.Text.Markdown
import Database.Persist.Sql

newtype MarkdownNewT = MarkdownNewT { getMarkdown :: Markdown }
  deriving (Eq, IsString, Monoid, PersistField, PersistFieldSql)

对于PersistFieldSql以下消息,此操作失败:

Could not coerce from ‘m Markdown’ to ‘m MarkdownNewT’
  because ‘m Markdown’ and ‘m MarkdownNewT’ are different types.
  arising from the coercion of the method ‘sqlType’ from type
               ‘forall (m :: * -> *). Monad m => m Markdown -> SqlType’ to type
               ‘forall (m :: * -> *). Monad m …

所以,我开始尝试quasiquotation和模板haskell.

我想修改一个现有的(大)准规则代码,同时使用在"被调用"的地方定义的变量的实际值.举一个简单的例子说明:

main.hs

{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE TemplateHaskell #-}

import Language.Haskell.TH
import Exp02

x = "cde"

main = do
  putStrLn [str|$x|]

Exp02.hs

{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE TemplateHaskell #-}

module Exp02 where

import Language.Haskell.TH
import Language.Haskell.TH.Syntax
import Language.Haskell.TH.Quote

xpto :: String -> ExpQ
xpto [] = stringE []
xpto ('$':rest) = varE (mkName rest)
xpto str = stringE str

str = QuasiQuoter
  { quoteExp = xpto
  , quotePat = fail $ "patterns"
  , quoteType= fail $ "types" …