the trivial monad- thoughts and a question
So, I've been playing around with what I call the trivial monad: module TrivialMonad where data TrivialMonad a = M a recover :: TrivialMonad a -> a recover (M x) = x instance Monad TrivialMonad where (M x) >>= f = f x (M x) >> f = f return x = M x fail s = undefined This is actually a surprisingly usefull little sucker- it allows you to "demonadify" code. Which is usefull when you have some places where you want to use the code within a monad, and some places where you don't. You write the base bit of code monadically, and then demonadify it as needed. The first question I have is it is possible to implement this guy without wrapping the value in a constructor? What I'd like to do is replace the: data TrivialMonad a = M a with something like: type TrivialMonad a = a and then be able to junk the recover function. The second question I have is: is there any hope of getting something like this into the standard library? Thanks, Brian
Brian Hurt wrote:
The second question I have is: is there any hope of getting something like this into the standard library?
the newtype Identity in module Control.Monad.Identity in package `mtl` is what you describe: http://www.haskell.org/ghc/docs/latest/html/libraries/mtl/Control-Monad-Iden... in other words, it already is practically in the standard library. The type-synonym technique doesn't work very well because, e.g., is (TrivialMonad (IO ())) ">>=" an IO function or a Trivial function? It has been thoroughly discussed before, IIRC... ~Isaac
On Jan 13, 2008 12:47 AM, Brian Hurt
So, I've been playing around with what I call the trivial monad:
module TrivialMonad where
data TrivialMonad a = M a
Better to use newtype here; then it really is operationally equivalent to using just "a", except that it's possible to implement a monad instance for it (see below).
The first question I have is it is possible to implement this guy without wrapping the value in a constructor? What I'd like to do is replace the: data TrivialMonad a = M a with something like: type TrivialMonad a = a and then be able to junk the recover function.
Nope. That would mean that every type is a monad, which would cause endless troubles for type inference. For example, what monad is (putStrLn "x") a member of: IO () or TrivialMonad (IO ()) (or even TrivialMonad (TrivialMonad (IO())))?
The second question I have is: is there any hope of getting something like this into the standard library?
Control.Monad.Identity, perhaps? http://www.haskell.org/ghc/docs/latest/html/libraries/mtl/Control-Monad-Iden... Luke
Am Sonntag, 13. Januar 2008 01:47 schrieb Brian Hurt:
So, I've been playing around with what I call the trivial monad:
module TrivialMonad where
data TrivialMonad a = M a
recover :: TrivialMonad a -> a recover (M x) = x
instance Monad TrivialMonad where
(M x) >>= f = f x (M x) >> f = f return x = M x fail s = undefined
This is actually a surprisingly usefull little sucker- it allows you to "demonadify" code. Which is usefull when you have some places where you want to use the code within a monad, and some places where you don't. You write the base bit of code monadically, and then demonadify it as needed.
The first question I have is it is possible to implement this guy without wrapping the value in a constructor? What I'd like to do is replace the: data TrivialMonad a = M a with something like: type TrivialMonad a = a and then be able to junk the recover function.
No, but you can use a newtype instead of data, so you have no run-time overhead.
The second question I have is: is there any hope of getting something like this into the standard library?
It's already there: Control.Monad.Identity
Thanks, Brian
Cheers, Daniel
participants (5)
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Brian Hurt -
Daniel Fischer -
Isaac Dupree -
Luke Palmer -
Miguel Mitrofanov