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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