data constructor with IO type
Hi All, I'd like to know does it exist a method which allow to construct the TestB type without a temporary variable such as the TestA constructor. --- module Main where import Monad import Control.Concurrent data TestA = TestA (MVar Bool) xGetA (TestA x) = x data TestB = TestB { xGetB :: MVar Bool } --- main = do tmp <- newMVar False let t = TestB { xGetB = tmp } in do vt <- takeMVar t putStrLn ("HELLO" ++ show vt) --- main = do t <- liftM TestA (newMVar False) vt <- takeMVar (xGetA t) putStrLn ("HELLO" ++ show vt) --- Daneel Yaitskov
On Tue, Feb 24, 2009 at 08:04:33PM +0300, Daneel Yaitskov wrote:
Hi All,
I'd like to know does it exist a method which allow to construct the TestB type without a temporary variable such as the TestA constructor.
--- module Main where
import Monad import Control.Concurrent data TestA = TestA (MVar Bool) xGetA (TestA x) = x data TestB = TestB { xGetB :: MVar Bool }
--- main = do tmp <- newMVar False let t = TestB { xGetB = tmp } in do vt <- takeMVar t putStrLn ("HELLO" ++ show vt) ---
main = do t <- liftM TestA (newMVar False) vt <- takeMVar (xGetA t) putStrLn ("HELLO" ++ show vt)
---
Daneel Yaitskov
Hi Daneel, Note that record syntax like
data TestB = TestB { xGetB :: MVar Bool }
only adds capabilities; you can still use TestB as if it was defined without record syntax, like TestA. So you are not required to use the TestB {xGetB = tmp} syntax to create a TestB, you could also just say 'TestB tmp'. So the second code example should work fine if you just replace all the 'A's with 'B's. Does this answer your question? I must admit that I am not entirely sure what you are asking, so if this doesn't address your question feel free to clarify. -Brent
Brent Yorgey
Note that record syntax like
data TestB = TestB { xGetB :: MVar Bool }
only adds capabilities; you can still use TestB as if it was defined without record syntax, like TestA. So you are not required to use the
TestB {xGetB = tmp}
syntax to create a TestB, you could also just say 'TestB tmp'. So the second code example should work fine if you just replace all the 'A's with 'B's.
Does this answer your question? I must admit that I am not entirely sure what you are asking, so if this doesn't address your question feel free to clarify.
-Brent
I know about it. I mean how to save names of arguments. Because usually data structures is complex and they contain many fields. Method which doesn't the names of the arguments requires to give the values for all members. Sometimes some subsets of one structure calculate themselves at other places or structure contains many fields. One part of them get themselves from the pure functions and other do from the action functions. Daneel Yaitskov
participants (2)
-
Brent Yorgey -
Daneel Yaitskov