Haskell dynamically set record field based on field name string?

Question

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}

Answer 1

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)