考虑 Haskell 的Generic课程：
class Generic a where
-- | Generic representation type
type Rep a :: * -> *
-- | Convert from the datatype to its representation
from :: a -> (Rep a) x
-- | Convert from the representation to the datatype
to :: (Rep a) x -> a
我很好奇为什么它没有写成如下：
class Generic a where
-- | Generic representation type
type Rep a :: *
-- | Convert from the datatype to its representation
from :: a …为什么这样做：
val somePair: Option[(String,String)] = Some(("John", "Doe"))
(for {
pair <- somePair.toRight("Hello unknown!").right
} yield s"Hello ${pair._1} ${pair._2}!").merge
但这不会：
val somePair: Option[(String,String)] = Some(("John", "Doe"))
(for {
(name,lastName) <- somePair.toRight("Hello unknown!").right
} yield s"Hello $name $lastName!").merge
编辑：
我应该添加这是错误消息：
Error:(43, 4) constructor cannot be instantiated to expected type;
found : (T1, T2)
required: scala.util.Either[Nothing,(String, String)]
(name,lastName) <- somePair.toRight("Hello unknown!").right
^
在我实现类型级编码树的早期阶段，当涉及类型约束时遇到不明确的类型时，我遇到了 GHC 在其类型推断中的特殊行为。我写了两个 AST 节点，如下所示，它们都可以通过它们实现的Typed类型类实例检查它们的类型：
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE FlexibleContexts #-}
class Typed t where
type T t
-- | A Literal node
newtype Lit a =
Lit a
instance Typed (Lit a) where
type T (Lit a) = a
-- | A Plus Node
data Plus a b =
Plus a b
instance T a ~ T b => Typed (Plus a b) where
type T (Plus a b) = T a
然后我编写了未类型检查的badPlus函数，它不对 …
在Scala中,"部分功能"的概念与F#的function关键字允许我实现的功能非常相似.然而,Scala的部分函数也允许通过orElse如下所示的方法进行合成:
def intMatcher: PartialFunction[Any,String] = {
case _ : Int => "Int"
}
def stringMatcher: PartialFunction[Any,String] = {
case _: String => "String"
}
def defaultMatcher: PartialFunction[Any,String] = {
case _ => "other"
}
val msgHandler =
intMatcher
.orElse(stringMatcher)
.orElse(defaultMatcher)
msgHandler(5) // yields res0: String = "Int"
我需要知道是否有办法在F#中实现相同的组合功能.
如何在默认情况下附带 python 2.7 的 amazon/aws-cli docker 映像上安装 Python 3.9？到目前为止，我尝试过的是基于在 Amazon Linux 2 上安装 Python 3 的安装教程的以下一系列命令：
FROM amazon/aws-cli:latest
WORKDIR ./
COPY ./.aws ./root/.aws
COPY . .
RUN yum install gcc openssl-devel bzip2-devel libffi-devel -y
RUN yum install wget tar -y
RUN cd /opt
RUN wget https://www.python.org/ftp/python/3.9.6/Python-3.9.6.tgz
RUN tar xzf Python-3.9.6.tgz
RUN cd Python-3.9.6
RUN ./configure --enable-optimizations
RUN make altinstall
RUN rm -f /opt/Python-3.9.6.tgz
RUN ["python3", "-u", "app.py"]
但我收到此错误：
[+] Building 13.5s (14/19)
=> [internal] load build definition from Dockerfile 0.1s …