Function for working with functions in data?
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.
I couldn't turn it up in hoogle or the prelude. Am I just missing it
somewhere, or is it really not available in a library?
Thanks,
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 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] - -- brandon s. allbery [linux,solaris,freebsd,perl] allbery.b@gmail.com system administrator [openafs,heimdal,too many hats] kf8nh -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (Darwin) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk3ESD0ACgkQIn7hlCsL25XFeQCgnD2GTT4sjYKJ5Nw9QGG+qGJb 8NkAoKq07s8sUxjqw3CthG/0HNiKwnlU =ifKQ -----END PGP SIGNATURE-----
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.
participants (3)
-
Brandon S Allbery KF8NH -
Daniel Fischer -
Mike Meyer