On Friday 06 May 2011 21:13:01, Brandon S Allbery KF8NH wrote:
On 5/6/11 14:57 , Mike Meyer wrote:
I'm filling a functional itch that Haskell isn't scratching. I'm looking for a function to take a list of functions, and apply them all to a value. The signature is obvious :: [a -> b] -> a -> [b], and it's nearly trivial to write (\fs a -> map ($ a) [fs]), but Haskell has most useful list functions already provided.
It's there, but it's not a dedicated function; it's "sequence" applied to the environment monad (-> e).
Prelude Control.Monad.Reader> :t sequence sequence :: (Monad m) => [m a] -> m [a] Prelude Control.Monad.Reader> :t sequence [(+1),(*2),(`subtract` 3)] sequence [(+1),(*2),(`subtract` 3)] :: (Num a) => a -> [a]
The Monad instance for ((->) e) is defined in Control.Monad.Instances, so to use it, you must directly or indirectly (e.g. via Control.Monad.Reader or Control.Monad.State) import that.