在Haskell中使用涉及CmpNat和Singleton的证明
我正在尝试创建一些函数来处理以下类型。以下代码使用GHC-8.4.1中的和库:在Haskell中使用涉及CmpNat和Singleton的证明,haskell,constraints,type-families,type-level-computation,singleton-type,Haskell,Constraints,Type Families,Type Level Computation,Singleton Type,我正在尝试创建一些函数来处理以下类型。以下代码使用GHC-8.4.1中的和库: {-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE PolyKinds #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE UndecidableInstances #-}
import Data.Constraint ((:-))
import Data.Singletons (sing)
import Data.Singletons.Prelude (Sing(SEQ, SGT, SLT), (%+), sCompare)
import Data.Singletons.Prelude.Num (PNum((+)))
import Data.Singletons.TypeLits (SNat)
import GHC.TypeLits (CmpNat, Nat)
data Foo where
Foo
:: forall (index :: Nat) (len :: Nat).
(CmpNat index len ~ 'LT)
=> SNat index
-> SNat len
-> Foo
这是一个包含长度和索引的GADT。保证索引小于长度
编写函数来创建Foo
,非常简单:
createFoo :: Foo
createFoo = Foo (sing :: SNat 0) (sing :: SNat 1)
但是,我在编写一个函数时遇到了问题,该函数在使索引保持不变的情况下增加len
:
incrementLength :: Foo -> Foo
incrementLength (Foo index len) = Foo index (len %+ (sing :: SNat 1))
此操作失败,出现以下错误:
file.hs:34:34: error:
• Could not deduce: CmpNat index (len GHC.TypeNats.+ 1) ~ 'LT
arising from a use of ‘Foo’
from the context: CmpNat index len ~ 'LT
bound by a pattern with constructor:
Foo :: forall (index :: Nat) (len :: Nat).
(CmpNat index len ~ 'LT) =>
SNat index -> SNat len -> Foo,
in an equation for ‘incrementLength’
at what5.hs:34:17-29
• In the expression: Foo index (len %+ (sing :: SNat 1))
In an equation for ‘incrementLength’:
incrementLength (Foo index len)
= Foo index (len %+ (sing :: SNat 1))
• Relevant bindings include
len :: SNat len (bound at what5.hs:34:27)
index :: SNat index (bound at what5.hs:34:21)
|
34 | incrementLength (Foo index len) = Foo index (len %+ (sing :: SNat 1))
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
这是有意义的,因为编译器知道CmpNat索引len~'LT
(根据Foo的定义),但不知道CmpNat索引(len+1)~'LT
有没有办法让这样的东西起作用
可以使用sCompare
执行以下操作:
incrementLength :: Foo -> Foo
incrementLength (Foo index len) =
case sCompare index (len %+ (sing :: SNat 1)) of
SLT -> Foo index (len %+ (sing :: SNat 1))
SEQ -> error "not eq"
SGT -> error "not gt"
然而,不幸的是,我不得不为SEQ
和SGT
编写案例,而我知道它们永远不会匹配
此外,我认为可以创建如下类型:
plusOneLTProof :: (CmpNat n m ~ 'LT) :- (CmpNat n (m + 1) ~ 'LT)
plusOneLTProof = undefined
file.hs:40:8: error:
• Couldn't match type ‘CmpNat n0 m0’ with ‘CmpNat n m’
Expected type: (CmpNat n m ~ 'LT) :- (CmpNat n (m + 1) ~ 'LT)
Actual type: (CmpNat n0 m0 ~ 'LT) :- (CmpNat n0 (m0 + 1) ~ 'LT)
NB: ‘CmpNat’ is a non-injective type family
The type variables ‘n0’, ‘m0’ are ambiguous
• In the ambiguity check for ‘bar’
To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
In the type signature:
bar :: (CmpNat n m ~ 'LT) :- (CmpNat n (m + 1) ~ 'LT)
|
40 | bar :: (CmpNat n m ~ 'LT) :- (CmpNat n (m + 1) ~ 'LT)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
incrementLength :: Foo -> Foo
incrementLength (Foo index len) =
case plusOneLTProof index len of
Sub Dict -> Foo index (len %+ (sing :: SNat 1))
plusOneLTProof :: forall n m. SNat n -> SNat m -> (CmpNat n m ~ 'LT) :- (CmpNat n (m + 1) ~ 'LT)
plusOneLTProof SNat SNat = Sub axiom
where
axiom :: CmpNat n m ~ 'LT => Dict (CmpNat n (m + 1) ~ 'LT)
axiom = unsafeCoerce (Dict :: Dict (a ~ a))
但是,这会产生如下错误:
plusOneLTProof :: (CmpNat n m ~ 'LT) :- (CmpNat n (m + 1) ~ 'LT)
plusOneLTProof = undefined
file.hs:40:8: error:
• Couldn't match type ‘CmpNat n0 m0’ with ‘CmpNat n m’
Expected type: (CmpNat n m ~ 'LT) :- (CmpNat n (m + 1) ~ 'LT)
Actual type: (CmpNat n0 m0 ~ 'LT) :- (CmpNat n0 (m0 + 1) ~ 'LT)
NB: ‘CmpNat’ is a non-injective type family
The type variables ‘n0’, ‘m0’ are ambiguous
• In the ambiguity check for ‘bar’
To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
In the type signature:
bar :: (CmpNat n m ~ 'LT) :- (CmpNat n (m + 1) ~ 'LT)
|
40 | bar :: (CmpNat n m ~ 'LT) :- (CmpNat n (m + 1) ~ 'LT)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
incrementLength :: Foo -> Foo
incrementLength (Foo index len) =
case plusOneLTProof index len of
Sub Dict -> Foo index (len %+ (sing :: SNat 1))
plusOneLTProof :: forall n m. SNat n -> SNat m -> (CmpNat n m ~ 'LT) :- (CmpNat n (m + 1) ~ 'LT)
plusOneLTProof SNat SNat = Sub axiom
where
axiom :: CmpNat n m ~ 'LT => Dict (CmpNat n (m + 1) ~ 'LT)
axiom = unsafeCoerce (Dict :: Dict (a ~ a))
我想这是有道理的,因为CmpNat是非内射的。但是,我知道这是真的,所以我希望能够编写这个函数
我想回答以下两个问题:
有没有一种方法可以编写incrementLength
,只需在SLT
上进行匹配?我可以修改Foo
的定义,使之更简单
有没有一种方法可以编写plusOnline
,或者至少是类似的东西
更新:我最终根据李耀霞的建议编写了plusInEltProof
和incrementLength
,如下所示:
plusOneLTProof :: (CmpNat n m ~ 'LT) :- (CmpNat n (m + 1) ~ 'LT)
plusOneLTProof = undefined
file.hs:40:8: error:
• Couldn't match type ‘CmpNat n0 m0’ with ‘CmpNat n m’
Expected type: (CmpNat n m ~ 'LT) :- (CmpNat n (m + 1) ~ 'LT)
Actual type: (CmpNat n0 m0 ~ 'LT) :- (CmpNat n0 (m0 + 1) ~ 'LT)
NB: ‘CmpNat’ is a non-injective type family
The type variables ‘n0’, ‘m0’ are ambiguous
• In the ambiguity check for ‘bar’
To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
In the type signature:
bar :: (CmpNat n m ~ 'LT) :- (CmpNat n (m + 1) ~ 'LT)
|
40 | bar :: (CmpNat n m ~ 'LT) :- (CmpNat n (m + 1) ~ 'LT)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
incrementLength :: Foo -> Foo
incrementLength (Foo index len) =
case plusOneLTProof index len of
Sub Dict -> Foo index (len %+ (sing :: SNat 1))
plusOneLTProof :: forall n m. SNat n -> SNat m -> (CmpNat n m ~ 'LT) :- (CmpNat n (m + 1) ~ 'LT)
plusOneLTProof SNat SNat = Sub axiom
where
axiom :: CmpNat n m ~ 'LT => Dict (CmpNat n (m + 1) ~ 'LT)
axiom = unsafeCoerce (Dict :: Dict (a ~ a))
这要求您将两个SNat
s传递给plusOneLTProof
,但它不需要AllowAmbiguousTypes
编译器拒绝plusOneLTProof
,因为它的类型不明确。我们可以使用扩展名AllowAmbiguousTypes
禁用该约束。我建议将其与ExplicitForall
(这是由ScopedTypeVariables
暗示的,我们肯定需要它,或者RankNTypes
)。这是为了定义它。类型不明确的定义可与TypeApplications
一起使用
然而,GHC仍然无法对自然进行推理,因此我们无法定义plusOnly=Sub-Dict
,更不用说incrementLength
,这是不安全的
但是我们仍然可以用不安全的力量
凭空创造证据。这实际上就是约束中模块的实现方式;不幸的是,它目前没有包含关于CmpNat
的任何事实。强制生效是因为类型equalities中没有运行时内容。即使运行时值看起来不错,因此断言不一致的事实仍然可能导致程序出错
plusOneLTProof :: forall n m. (CmpNat n m ~ 'LT) :- (CmpNat n (m+1) ~ 'LT)
plusOneLTProof = Sub axiom
where
axiom :: (CmpNat n m ~ 'LT) => Dict (CmpNat n (m+1) ~ 'LT)
axiom = unsafeCoerce (Dict :: Dict (a ~ a))
为了使用证明,我们将其专门化(使用TypeApplications
)并在其上进行模式匹配,以便在上下文中引入RHS,检查LHS是否成立
incrementLength :: Foo -> Foo
incrementLength (Foo (n :: SNat n) (m :: SNat m)) =
case plusOneLTProof @n @m of
Sub Dict -> Foo n (m %+ (sing :: SNat 1))