Haskell type constraints unleashed - Orchard, Schrijvers - 2010 () Citation Context ...ry [34]. 3.1 Constraint Kinds Constraint kinds were implemented by Bolingbroke [3], motivated by a desire to support the constraint synonyms and constraint families proposed by Orchard and Schrijvers =-=[29]-...
transferring familiar type-level constructs,synonymsandfamilies, to the language of constraints, providing a symmetrical set of features at the type-level and constraint-level. We introduceconstraint synonymsandconstraint families, and illustrate their increased expressivity for improving the utility of polym...
<name> :: <type constraint A> => -> -> .. -> <return type> Run Code Online (Sandbox Code Playgroud) 因此,以我目前的理解为例:-- Returns input + 2 add2 :: Int -> Int add2 x = x + 2 -- Returns the result of applying a function ...
type IsGenerative :: (Type -> Type) -> Constraint class (forall g a b. f a ~ g b => f ~ g) => IsGenerative f 相反,用文字表述: 如果除了F之外没有G :: Type -> Type存在,使得存在A,B :: Type,使得F A ~ G B,则F :: Type -> Type是生成的。 该论文继续阐述了非饱和类型族的...
getChar是一个从输入读进一个字元的 I/O action,因此他的 type signature 是getChar :: IO Char,代表一个 I/O action 的结果是Char。注意由于缓冲区的关系,只有当 Enter 被按下的时候才会触发读取字元的行为。 main =doc <- getCharifc /=' 'thendoputChar c ...
instance CHRV [Int]wheretypeConstraint [Int] = List Int instance CHR [Int]test:: [Int] -> Booltestxs = runCHR $dorule1 Nil xs xs rule2 xs main :: IO () main =print$test[1,2,3] AI代码助手复制代码 LogicT:LogicT是一个monad变换器,允许将非确定性选择操作和逻辑变量引入Haskell中。通...
Ambiguous type variable `a0' in the constraint: (Read a0) arising from a use of `read' Probable fix: add a type signature that fixes these type variable(s) In the expression: read "5" In an equation for `it': it = read "5" ...
P.S.也可以对着typeclass来一发,例如: 代码语言:javascript 代码运行次数:0 运行 AI代码解释 > :k Functor Functor :: (* -> *) -> Constraint > :k Eq Eq :: * -> Constraint 其中Constraint也是一种kind,表示必须是某类的instance(即类型约束,经常在函数签名的=>左边看到),例如Num,具体见What does...
weirdID::∀a.Typeablea=>a->aweirdID|JustHRefl<-typeRep@a`eqTypeRep`typeRep@Bool=\caseTrue->Falsex->x|otherwise=\x->x 不难看出,这里在运行期比较了a和Bool是否相等,根据比较结果来决定函数行为。Typeableconstraint 给了类型 Propositional equality 的能力。weirdID中的a已经算是dependent & releva...
type Foo a = Num a => a data Bar a = Bar (Foo a) -- equals "data Bar a = Bar (Num a => a)" -- requires ExistentialQuantifiaction or GADTs to compile -- and of course, any data constructor blocks the outwards floating of the constraint: type Baz = a -> (Foo a, ()) ...