Haskell根据字段名字符串动态设置记录字段？ [英] Haskell dynamically set record field based on field name string?
问题描述
data Rec = Rec {
field1 :: Int，
field2 :: Int
}
如何编写函数：
changeField :: Rec - >字符串 - > Int - > Rec
changeField rec fieldName value
这样我就可以传递字符串field1或field2放入 fieldName 参数中并让它更新相关字段？我理解 Data.Data 和 Data.Typeable 是在这里使用的，但我无法将这两个包。
我见过的库的一个例子是cmdArgs。以下是关于如何使用此库的博客文章的摘录 ：
{ - ＃LANGUAGE DeriveDataTypeable＃ - }
import System.Console.CmdArgs
数据Guess =猜测{min :: Int，max :: Int，limit :: Maybe Int}派生（Data，Typeable，Show）
main = do
x< - cmdArgs $ Guess 1 100 Nothing
print x
现在我们有一个简单的命令行解析器。一些示例交互是：
$ guess --min = 10
NumberGuess {min = 10，max = 100，限制=无}}
我实现了一个更通用的修改字段，并且使用模板haskell，或者要求您手动管理字段映射。它接受一个mutator函数，并使用它与 const值一起实现 setField （nee changeField >） 。
$ b
modifyField 和 setField 在record和mutator / value类型中都是通用的;然而，为了避免 Num 歧义，调用示例中的数字常量必须显式给出 :: Int 签名。
我也改变了参数顺序，所以 rec 最后出现，允许一个修改字段 / setField 由普通函数组合创建（参见上一个调用示例）。
modifyField 建立在基元 gmapTi 之上，它是 Data.Data 。它是 gmapT 和 gmapQi 之间的交叉点。
<$ p $ b $ {$＃$ c $ {$＃$ $ $ $ $ $ $ $'$ $ $ $ $ $'$ $ $ $'导入Data.Data（Data，gfoldl，gmapQi，ConstrRep（AlgConstr），
toConstr，constrRep，constrFields）
导入Data.Generics（extT，extQ）
导入Data.List（elemIndex）
导入Control.Arrow（（&&&））
数据Rec = Rec {
field1 :: Int，
field2 :: String
$ r $ = $ {$ field $ b让r'= setFieldfield1（10 :: Int）r
print r'
let r''= setFieldfield2worldr'
print r''
印刷。 modifyFieldfield1（succ :: Int - > Int）。 setFieldfield2there$ r
print（getFieldfield2r':: String）
----------------- -------------------------------------------------- --------------------
data Ti a = Ti Int a
gmapTi :: Data a => ; Int - > （全部b数据b => b→b）→> a - > a
gmapTi i f x = {Ti _ a - >> a}
其中
k :: Data d => Ti（d-> b） - > d - >如果i == i，那么c（f a）else c a）
z :: g - > Ti g
z = Ti 0
--------------------------------- -------------------------------------------------- ----
fieldNames ::（Data r）=> r - > [String]
fieldNames rec =
case（constrRep&&&constrFields）$ toConstr rec of
（AlgConstr _，fs）|不是$ null fs - > fs
其他 - >错误不是记录类型
fieldIndex ::（Data r）=>字符串 - > r - > Int
fieldIndex fieldName rec =
case fieldName`elemIndex` fieldNames rec of
Just i - >我
无 - > $ fieldName
modifyField ::（Data r，Typeable v）=>字符串 - > （v→v）→> r - > r
modifyField fieldName m rec = gmapTi i（e`extT` m）rec
其中
i = fieldName`fieldIndex` rec
ex = error $类型不匹配：++ fieldName ++
::++（show。typeOf $ x）++
，而不是++（show。typeOf $ m undefined）
setField :: （Data r，Typeable v）=>字符串 - > v - > r - > r
setField fieldName value = modifyField fieldName（const value）
$ b $ getField ::（Data r，Typeable v）=>字符串 - > r - > v
getField fieldName rec = gmapQi i（e`extQ` id）rec
其中
i = fieldName`fieldIndex` rec
ex = error $类型不匹配：++ fieldName + +
::++（show。typeOf $ x）++
，not++（show。typeOf $ e undefined）
Say I have the following record:
data Rec = Rec {
field1 :: Int,
field2 :: Int
}
How do I write the function:
changeField :: Rec -> String -> Int -> Rec
changeField rec fieldName value
such that I can pass in the strings "field1" or "field2" into the fieldName argument and have it update the associated field? I understand Data.Data and Data.Typeable are what to use here but I can't figure these two packages out.
An example of a library I've seen do this is cmdArgs. Below is an excerpt from a blog posting on how to use this library:
{-# LANGUAGE DeriveDataTypeable #-}
import System.Console.CmdArgs
data Guess = Guess {min :: Int, max :: Int, limit :: Maybe Int} deriving (Data,Typeable,Show)
main = do
x <- cmdArgs $ Guess 1 100 Nothing
print x
Now we have a simple command line parser. Some sample interactions are:
$ guess --min=10
NumberGuess {min = 10, max = 100, limit = Nothing}
解决方案 OK, here's a solution that doesn't use template haskell, or require you to manage the field map manually.
I implemented a more general modifyField which accepts a mutator function, and implemented setField (nee changeField) using it with const value.
The signature of modifyField and setField is generic in both the record and mutator/value type; however, in order to avoid Num ambiguity, the numeric constants in the invocation example have to be given explicit :: Int signatures.
I also changed the parameter order so rec comes last, allowing a chain of modifyField/setField to be created by normal function composition (see the last invocation example).
modifyField is built on top of the primitive gmapTi, which is a 'missing' function from Data.Data. It is a cross between gmapT and gmapQi.
{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE RankNTypes #-}
import Data.Typeable (Typeable, typeOf)
import Data.Data (Data, gfoldl, gmapQi, ConstrRep(AlgConstr),
toConstr, constrRep, constrFields)
import Data.Generics (extT, extQ)
import Data.List (elemIndex)
import Control.Arrow ((&&&))
data Rec = Rec {
field1 :: Int,
field2 :: String
} deriving(Show, Data, Typeable)
main = do
let r = Rec { field1 = 1, field2 = "hello" }
print r
let r' = setField "field1" (10 :: Int) r
print r'
let r'' = setField "field2" "world" r'
print r''
print . modifyField "field1" (succ :: Int -> Int) . setField "field2" "there" $ r
print (getField "field2" r' :: String)
---------------------------------------------------------------------------------------
data Ti a = Ti Int a
gmapTi :: Data a => Int -> (forall b. Data b => b -> b) -> a -> a
gmapTi i f x = case gfoldl k z x of { Ti _ a -> a }
where
k :: Data d => Ti (d->b) -> d -> Ti b
k (Ti i' c) a = Ti (i'+1) (if i==i' then c (f a) else c a)
z :: g -> Ti g
z = Ti 0
---------------------------------------------------------------------------------------
fieldNames :: (Data r) => r -> [String]
fieldNames rec =
case (constrRep &&& constrFields) $ toConstr rec of
(AlgConstr _, fs) | not $ null fs -> fs
otherwise -> error "Not a record type"
fieldIndex :: (Data r) => String -> r -> Int
fieldIndex fieldName rec =
case fieldName `elemIndex` fieldNames rec of
Just i -> i
Nothing -> error $ "No such field: " ++ fieldName
modifyField :: (Data r, Typeable v) => String -> (v -> v) -> r -> r
modifyField fieldName m rec = gmapTi i (e `extT` m) rec
where
i = fieldName `fieldIndex` rec
e x = error $ "Type mismatch: " ++ fieldName ++
" :: " ++ (show . typeOf $ x) ++
", not " ++ (show . typeOf $ m undefined)
setField :: (Data r, Typeable v) => String -> v -> r -> r
setField fieldName value = modifyField fieldName (const value)
getField :: (Data r, Typeable v) => String -> r -> v
getField fieldName rec = gmapQi i (e `extQ` id) rec
where
i = fieldName `fieldIndex` rec
e x = error $ "Type mismatch: " ++ fieldName ++
" :: " ++ (show . typeOf $ x) ++
", not " ++ (show . typeOf $ e undefined)
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