记录的字段可以与某个值匹配并同时分配给一个参数？ [英] Can record's field be matched with some value and assigned to a parameter at the same time?

问题描述

在Haskell维基上关于模式匹配记录的相对简短的文字之后，我不确定记录的字段是否可以与某个值匹配并同时分配给参数。

   - 样本记录类型
数据Rec = Rec {uid :: Int，name :: String} 
 
  - 函数更新Rec的字段
 updateRec :: Rec  - > Rec 
 
  - 与记录数据构造函数进行基本匹配
 updateRec（Rec _ []）= Rec 0Nobody
 
  -  dtto with as-pattern at第二个参数
 updateRec（Rec idn ns @Alice）= Rec（idn + 1）（ns +++1）
 
  - 通过字段值进行基本匹配
 updateRec Rec {name =Bob，uid = 42} = Rec（10 ^ 6）SuperBob
 
  - 问：如何通过名称字段匹配并赋值？ 
 updateRec Rec {name =Bob，uid = idn} = 
 Rec {uid =（idn + 1），name =（Bob+++1）} 
 
  -  Q：as-pattern只适用于整个记录，不适用于字段
 updateRec rec @（Rec {name =Bob，uid = idn}）= 
 Rec { uid =（idn + 1），name =（name rec +++1）} 
 
  -  Q：模式守卫可以工作，但它们有点笨拙
 updateRec Rec {name = uname，uid = idn} 
 | uname ==Bob= Rec {uid =（idn + 1），name =（uname +++1）} 
 |否则= ... 
  

那么是否有一些简洁的方式来按照记录的字段进行模式匹配并赋值

注意：我想避免具体的GHC扩展。

解决方案

<$> p $ p $ updateRec Rec {name = uname @Bob，uid = idn @ 42} = ...


没有更简洁的方式来做到这一点，标准或GHC扩展。当然，对于这样简单的情况，您可以简单地省略绑定并使用Bob和 42 直接在函数体中。

After a relatively brief text on Haskell wiki about pattern matching of records, I'm not sure if record's field can be matched with some value and assigned to a parameter at the same time.

-- Sample record type
data Rec = Rec { uid :: Int, name :: String }

-- Function to update Rec's fields
updateRec :: Rec -> Rec

-- basic matching with record data constructor
updateRec (Rec _ []) = Rec 0 "Nobody"

-- dtto with as-pattern at 2nd param
updateRec (Rec idn ns@"Alice") = Rec (idn+1) (ns ++ "+1")

-- basic matching by fields values
updateRec Rec {name = "Bob", uid = 42} = Rec (10^6) "SuperBob"

-- Q: how to match by name field and assign it's value ?
updateRec Rec {name = "Bob", uid = idn} =
    Rec {uid = (idn+1), name = ("Bob" ++ "+1")}

-- Q: as-pattern works only on the whole record, not on a field
updateRec rec@(Rec {name = "Bob", uid = idn}) =
    Rec {uid = (idn+1), name = (name rec ++ "+1")}

-- Q: pattern guards do work, but they are a bit clumsy
updateRec Rec {name = uname, uid = idn}
    | uname == "Bob" = Rec {uid = (idn+1), name = (uname ++ "+1")}
    | otherwise = ...

So is there some concise way to pattern match by record's field and assign its value at the same time?

Note: I would like to avoid specific GHC's extensions.

解决方案

The field names are patterns themselves, so you can use another as-pattern:

updateRec Rec {name = uname@"Bob", uid = idn@42} = ...

There isn't any more concise way to do this, standard or with GHC extensions. Of course, for a case as simple as this, you could simply omit the bindings and use "Bob" and 42 directly in the function body.

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