[Haskell-cafe] Monad transformer: apply StateT to List monad
michael rice
nowgate at yahoo.com
Fri Jan 14 01:38:31 CET 2011
Hi Daniel,
What I need to see is a function, say g, that lifts the function f (in the List monad) into the StateT monad, applies it to the monad's value, say 1, and returns a result [0,1].
Or, alternatively, code that lifts a function in the State monad, say tick
import Control.Monad.State
type GeneratorState = State Int
tick :: GeneratorState Int
tick = do n <- get
put (n+1)
return n
into the ListT monad and applies it to a list, say
lst = [0,1,2]
producing [(0,1),(1,2),(2,3)].
Both would be very helpful. Or maybe I'm missing the concept of monad transformers altogether and putting them together improperly, like trying to use a spreadsheet to write a letter?
Michael
--- On Thu, 1/13/11, Daniel Fischer <daniel.is.fischer at googlemail.com> wrote:
From: Daniel Fischer <daniel.is.fischer at googlemail.com>
Subject: Re: [Haskell-cafe] Monad transformer: apply StateT to List monad
To: haskell-cafe at haskell.org
Cc: "michael rice" <nowgate at yahoo.com>
Date: Thursday, January 13, 2011, 3:44 PM
On Thursday 13 January 2011 21:17:41, michael rice wrote:
> {- From: http://en.wikibooks.org/wiki/Haskell/Monad_transformers
> if for instance we apply StateT to the List monad, a function that
> returns a list (i.e., a computation in the List monad) can be lifted
> into StateT s [], where it becomes a function that returns a StateT (s
> -> [(a,s)]). That is, the lifted computation produces multiple
> (value,state) pairs from its input state. -}
>
> import Control.Monad.Trans.State.Lazy
>
> type GeneratorState = StateT Int
>
> -- a function in the list monad
> f :: Int -> [Int]
> f n = [0..n]
>
> Will someone please demonstrate the above comment from the wiki page.
lift (f n) = StateT (\s -> [(k,s) | k <- [0 .. n]])
Generally,
lift list = StateT (\s -> zip list (repeat s))
>
> Michael
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