[英]Why is my Semigroup/Monoid instance overlapping?
I'm trying to implement a way to lazily construct nondeterministic finite automata (NFAs).我正在尝试实现一种延迟构造非确定性有限自动机 (NFA) 的方法。 I did this years ago in F# and now want to try it with Haskell while leveraging the
Monoid
typeclass.我几年前在 F# 中做过这件事,现在想在利用
Monoid
类型类的同时尝试使用 Haskell。
{-# LANGUAGE TypeSynonymInstances, FlexibleInstances #-}
module NFA where
data State = State Match State | Split State State | Final deriving (Show)
data Match = Any | Char Char | ... deriving (Show)
type StateF = State -> State
complete :: StateF -> State -> State
complete statef exit = statef exit
connect :: StateF -> StateF -> StateF
connect fst snd = complete fst . complete snd
empty :: StateF
empty = id
instance Semigroup StateF where
(<>) = connect
instance Monoid StateF where
mempty = empty
This code doesn't compile, because my Semigroup
and Monoid
instances are overlapping with instance Semigroup b => Semigroup (a -> b)
and instance Monoid b => Monoid (a -> b)
from GHC.Base
, but I don't understand why.此代码无法编译,因为我的
Semigroup
和Monoid
实例与来自GHC.Base
的实例 Semigroup b => Semigroup (a instance Semigroup b => Semigroup (a -> b)
和instance Monoid b => Monoid (a -> b)
重叠,但我不这样做不明白为什么。
I see that there is a Monoid
instance on functions a -> b
, where b
is a Monoid
itself.我看到函数
a -> b
上有一个Monoid
实例,其中b
是Monoid
本身。 But State
doesn't have a Monoid
instance, so how can StateF
( State -> State
) overlap?但是
State
没有Monoid
实例,那么StateF
( State -> State
) 怎么重叠呢?
Is it because someone might implement Monoid
for State
elsewhere?是因为有人可能会在其他地方为
State
实现Monoid
吗?
Also, how can I fix this?另外,我该如何解决这个问题?
I'm aware that a could just define StateF
as...我知道 a 可以将
StateF
定义为...
data StateF = StateF (State -> State)
...but that would also increase syntax noise when pattern matching and constructing StateF
s. ...但这也会在模式匹配和构造
StateF
时增加语法噪音。
The comiler errors:编译器错误:
src\NFA.hs:10:10: error:
* Overlapping instances for Semigroup StateF
arising from a use of `GHC.Base.$dmsconcat'
Matching instances:
instance Semigroup b => Semigroup (a -> b) -- Defined in `GHC.Base'
instance Semigroup StateF -- Defined at src\NFA.hs:10:10
* In the expression: GHC.Base.$dmsconcat @(StateF)
In an equation for `GHC.Base.sconcat':
GHC.Base.sconcat = GHC.Base.$dmsconcat @(StateF)
In the instance declaration for `Semigroup StateF'
|
10 | instance Semigroup StateF where
| ^^^^^^^^^^^^^^^^
src\NFA.hs:10:10: error:
* Overlapping instances for Semigroup StateF
arising from a use of `GHC.Base.$dmstimes'
Matching instances:
instance Semigroup b => Semigroup (a -> b) -- Defined in `GHC.Base'
instance Semigroup StateF -- Defined at src\NFA.hs:10:10
* In the expression: GHC.Base.$dmstimes @(StateF)
In an equation for `GHC.Base.stimes':
GHC.Base.stimes = GHC.Base.$dmstimes @(StateF)
In the instance declaration for `Semigroup StateF'
|
10 | instance Semigroup StateF where
| ^^^^^^^^^^^^^^^^
src\NFA.hs:13:10: error:
* Overlapping instances for Semigroup StateF
arising from the superclasses of an instance declaration
Matching instances:
instance Semigroup b => Semigroup (a -> b) -- Defined in `GHC.Base'
instance Semigroup StateF -- Defined at src\NFA.hs:10:10
* In the instance declaration for `Monoid StateF'
|
13 | instance Monoid StateF where
| ^^^^^^^^^^^^^
src\NFA.hs:13:10: error:
* Overlapping instances for Monoid StateF
arising from a use of `GHC.Base.$dmmappend'
Matching instances:
instance Monoid b => Monoid (a -> b) -- Defined in `GHC.Base'
instance Monoid StateF -- Defined at src\NFA.hs:13:10
* In the expression: GHC.Base.$dmmappend @(StateF)
In an equation for `mappend':
mappend = GHC.Base.$dmmappend @(StateF)
In the instance declaration for `Monoid StateF'
|
13 | instance Monoid StateF where
| ^^^^^^^^^^^^^
src\NFA.hs:13:10: error:
* Overlapping instances for Monoid StateF
arising from a use of `GHC.Base.$dmmconcat'
Matching instances:
instance Monoid b => Monoid (a -> b) -- Defined in `GHC.Base'
instance Monoid StateF -- Defined at src\NFA.hs:13:10
* In the expression: GHC.Base.$dmmconcat @(StateF)
In an equation for `mconcat':
mconcat = GHC.Base.$dmmconcat @(StateF)
In the instance declaration for `Monoid StateF'
|
13 | instance Monoid StateF where
| ^^^^^^^^^^^^^
At least for the code shown, changing StateF
from a type alias to a newtype
introduces minimal changes and no runtime overhead.至少对于显示的代码,将
StateF
从类型别名更改为新类型newtype
引入最小的更改并且没有运行时开销。
module NFA where
data State = State Match State | Split State State | Final deriving (Show)
data Match = Any | Char Char | ... deriving (Show)
newtype StateF = StateF (State -> State)
-- This is one change
complete :: StateF -> State -> State
complete (StateF f) = f
connect :: StateF -> StateF -> StateF
connect fst snd = complete fst . complete snd
-- This is the other
empty :: StateF
empty = StateF id
instance Semigroup StateF where
(<>) = connect
instance Monoid StateF where
mempty = empty
If you use record syntax, you won't even need pattern-matching for complete
:如果你使用记录语法,你甚至不需要模式匹配来
complete
:
newtype StateF = StateF { runStateF :: State -> State }
complete :: StateF -> State -> State
-- complete statef exit = runStateF statef exit
-- complete statef = runStateF statef
complete = runStateF
(Don't think of complete
as actually applying the state transformer to a state, but rather extracting the state transformer so that it can be applied to a state.) (不要将
complete
视为实际将状态转换器应用于状态,而是提取状态转换器以便将其应用于状态。)
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