自动函子实例 [英] Automatic Functor Instance
问题描述
ghci> data Foo a = Foo a
然后我实例化其中的一个。
ghci>让f = Foofoo
最后,我想调用 fmap
将函数应用为,(a - > b) - > Foo a - > Foo b
。
ghci> fmap(++bar)f
< interactive>:78:1:
由于使用'fmap'而没有实例(Functor Foo)
In表达式:fmap(++bar)f
在等式'it'中:it = fmap(++bar)f
但是,因为我没有实现 Functor
Foo
,我不能使用 fmap
。
有没有办法获得 Functor
实例是免费的吗?我对Haskell的编译器知之甚少,但也许知道 Foo a
上的 fmap
仅仅适用于(a - > b)
至 Foo
的 a
?
在ghci中工作,如果你做了咒语
前奏> :set -XDeriveFunctor
然后编译器会变得像你所期望的那样聪明,热情。您将需要调用该功能,因此,
Prelude>数据Foo a = Foo a派生(Show,Functor)
( Show
仅用于打印输出,如下所示),然后您就可以执行诸如
前奏> fmap(++bar)(Foofoo)
Foofoobar
在一个模块中,通过添加附注来达到同样的效果
{ - #LANGUAGE DeriveFunctor# - }
在模块
声明之前。这对于至少更简单的 Functor
实例是很好的,但是你可以将它伪装成一个错误的否定。
前奏>数据Boo a = Boo(或者Bool)派生Functor
$ p $同时
< interactive>:9:43:
无法创建'Functor Boo'派生实例:
构造函数'Boo'必须仅使用类型变量作为数据类型的
最后一个参数
在'Boo'的数据声明中
数据Goo a = Goo(或者Bool a)派生Functor
没问题,并且机器显然已被黑客入侵,因为
data Woo a = Woo(a,Bool)派生Functor
是允许的。
所以它不像它那么聪明,但是它比眼中的捅。 p>
Given the following algebraic type:
ghci> data Foo a = Foo a
Then I instantiate one of them.
ghci> let f = Foo "foo"
Lastly, I'd like to call
fmap
to apply a function as,(a -> b) -> Foo a -> Foo b
.ghci> fmap (++ "bar") f <interactive>:78:1: No instance for (Functor Foo) arising from a use of ‘fmap’ In the expression: fmap (++ "bar") f In an equation for ‘it’: it = fmap (++ "bar") f
But, since I didn't implement a
Functor
instance ofFoo
, I can't usefmap
.Is there a way to get
Functor
instances for free? I have zero knowledge of Haskell's compiler, but perhaps it's clever enough to know thatfmap
onFoo a
is simply apply(a -> b)
toFoo
'sa
?解决方案Working in ghci, if you make the incantation
Prelude> :set -XDeriveFunctor
then the compiler will become as clever as you are hoping for, if not quite as enthusiastic. You will need to invoke the functionality, thus,
Prelude> data Foo a = Foo a deriving (Show, Functor)
(the
Show
is just for printing the output, below) and then you will be able to do stuff likePrelude> fmap (++"bar") (Foo "foo") Foo "foobar"
In a module, you achieve the same by adding the pragma
{-# LANGUAGE DeriveFunctor #-}
before the
module
declaration. It's good for at least the more straightforwardFunctor
instances, but you can fool it into a false negative.Prelude> data Boo a = Boo (Either a Bool) deriving Functor <interactive>:9:43: Can't make a derived instance of ‘Functor Boo’: Constructor ‘Boo’ must use the type variable only as the last argument of a data type In the data declaration for ‘Boo’
Meanwhile
data Goo a = Goo (Either Bool a) deriving Functor
is ok, and the machinery has clearly been hacked to work with pairing, as
data Woo a = Woo (a, Bool) deriving Functor
is permitted.
So it's not as clever as it could be, but it's better than a poke in the eye.
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