Generics can help. But they are much slower than pattern matching.
{-# LANGUAGE DeriveDataTypeable #-}
import Data.Generics
import Control.Monad.State
type A = ( Int, String )
data B = B Int String deriving ( Show, Typeable, Data )
f :: ( Typeable a, Data d ) => [ a ] -> d -> d
f s = changeField 2 ( s ++ )
changeField :: ( Typeable a, Num n, Data d ) => n -> ( a -> a ) -> d -> d
changeField num fun input = evalState ( gmapM f input ) 1
where
f a = do
x <- get
put $ x + 1
mkM ( \ a -> return $ if num == x then fun a else a ) a
--
*Main> f "asd" $ B 123 "dsa"
B 123 "asddsa"
*Main> f "asd" ( 123, "dsa" )
(123,"asddsa")
Alexey Karakulov ?????:
I wonder if pattern matching could be less verbose. Maybe this sounds
weird, but here is example of what I mean:
> type A = (Int, String)
>
> f :: String -> A -> A
> f s (i,s') = (i, s ++ s')
>
> data B = B Int String deriving Show
>
>g :: String -> B -> B
>g s (B i s') = B i $ s ++ s'
Types A/B and functions f/g are quite similar: (x :: A) or (x :: B)
means that x contains some integer and string values, and f/g
functions take some string and prepend it to the string part of x. The
code for f and g has the same level of verbosity, but -- ta-dah! -- we
can use arrows and define f in a highly laconic manner:
> import Control.Arrow
> f' :: String -> A -> A
> f' = second . (++)
So my queastion is how I could define (g' :: String -> B -> B) in the
same way.
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