记录的字段是否可以与某个值匹配，并同时分配给一个参数？

Question

在Haskell wiki上写了一篇关于记录模式匹配的相对简短的文章之后，我不确定record的字段是否可以与某个值匹配，并同时分配给一个参数。

-- 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 = ...

那么，是否有一些简洁的方法可以通过记录字段进行模式匹配，同时分配其值呢？

注意事项：我想避免特定的GHC扩展。

Answer 1

字段名本身就是模式，所以您可以使用另一个模式。

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

没有任何更简洁的方法来做到这一点，标准或使用GHC扩展。当然，对于这样简单的情况，您可以简单地省略绑定，直接在函数体中使用"Bob"和42。