加入函数和类型问题haskell [英] join function and type issues haskell

问题描述

我有2个列表表达式和绑定（id = Expr），并试图用一个名为newE的新列表中的绑定列表替换每个表达式，其中Expression = id ..
最初，我有只有一个表达式：

blockquote
eq（div（add 2 7）（sub 5 2））3
/ blockquote>

我想用该表达式中的每个标识符替换绑定列表中的每个标识符，所以我试图将这个表达式拆分为一个字符串列表并删除括号，以分隔每个标识符..

这是我试图实现它的方式：

  newE = [\x  - > getExp（b）| x < -  eStr，b < -  bs，x == getId（b）] 
其中es = getExpressions（prog）
 bs = getBindings（prog）
  - 一个字符串列表
  - 删除括号
 eStr = map（delete'）'）（map（delete'）'）
（map（delete'（'）（split （show es））））
 newE'= joinnewE 
  

这是否正确

现在我得到的错误是，newE返回[t0 - > Expr]，而它应该只返回Expr，为什么会这样？

，并且连接函数期望[Char] ..而它的类型在 Data.List.Utils 文档是：

join :: [a] - > [ [a]] - > [a]

所以，它不应该接受任何类型，而不仅仅是特征列表RS？还是与另一个图书馆的加入相混淆？
我搜索了我导入的库，但它们没有连接。

任何帮助解决这些错误并修改代码以执行它应该做什么？

谢谢

解决方案

，这是一个传统方法的草图：

1. 将字符串表达式转换为 Expr < code $ value，例如



   add 2 7 - > App（App（Varadd）（Var2））（Var7）


2. 编写一个函数 lookupBinding 来查找符号的绑定： b
   

   lookupBinding :: [Binding] - > Symbol - > Maybe Expr

3. 编写替代函数以将绑定定义替换为表达式：
   $ b

   substitute :: [Binding] - > Expr - > Expr

它会像这样：



 替代绑定（App e1 e2）= App e1'e2'
其中e1'=替代绑定e1 
 e2'=替换绑定e2 
 
替换绑定（Var sym）= ... lookupBinding sym ... 
替换绑定（Lam sym exp）= ...替代绑定'exp ... 
  




I have 2 lists Expressions and bindings (id = Expr), and trying to replace each expression with its equivalent from the bindings list in a new list called newE, where Expression = id .. Initially, I have only one expression:

eq (div (add 2 7) (sub 5 2)) 3

I want to replace each identifier in this expression with its equivalent from the bindings list, so I tried to split this expression into a list of strings and removed brackets, to separate each identifier ..

This is how I tried to implement it:

newE = [\x -> getExp(b) | x <- eStr, b <- bs, x == getId(b)]
                where es = getExpressions (prog)
                      bs = getBindings (prog)
                      -- Extracting expression into a list of strings 
                      -- and removing brackets
                      eStr = map (delete ')')(map (delete ')') 
                             (map (delete '(') (split " " (show es))))
newE' = join " " newE

Is this correct?

Now I'm getting errors that newE returns [t0 -> Expr] while it's supposed to return Expr only, why is that?

and the join function is expecting a [Char] .. while its type in the Data.List.Utils documentation is:

join :: [a] -> [[a]] -> [a]

so, isn't it supposed to accept any type not just list of characters? or did it get confused with a 'join' from another library? I searched the libraries I've imported, but they don't have a join.

Any help to resolve these errors and modify the code to do what it's supposed to do?

Thank you

解决方案

Since you asked, here is a sketch of the conventional approach:

1. Convert the string expression into a Expr value, e.g.

   add 2 7 -> App (App (Var "add") (Var "2")) (Var "7")

2. Write a function lookupBinding to lookup a binding for a Symbol:

   lookupBinding :: [Binding] -> Symbol -> Maybe Expr

3. Write a substitute function to substitute binding definitions into an expression:

   substitute :: [Binding] -> Expr -> Expr

It will go something like this:

substitute bindings (App e1 e2) = App e1' e2'
  where e1' = substitute bindings e1
        e2' = substitute bindings e2

substitute bindings (Var sym) = ... lookupBinding sym ...
substitute bindings (Lam sym exp) = ... substitute bindings' exp ...

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