Re: [Haskell-beginners] Beginners Digest, Vol 111, Issue 4

Would a specialization of return work, as in f = return :: Maybe a -> IO (Maybe a)? On Mon, Sep 4, 2017 at 7:00 AM, wrote:

Send Beginners mailing list submissions to beginners@haskell.org

To subscribe or unsubscribe via the World Wide Web, visit http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners or, via email, send a message with subject or body 'help' to beginners-request@haskell.org

You can reach the person managing the list at beginners-owner@haskell.org

When replying, please edit your Subject line so it is more specific than "Re: Contents of Beginners digest..."

Today's Topics:

1. I have a -> f a. How to get m a -> f m a ? (Baa) 2. Re: I have a -> f a. How to get m a -> f m a ? (Imants Cekusins)

----------------------------------------------------------------------

Message: 1 Date: Mon, 4 Sep 2017 15:06:34 +0300 From: Baa To: beginners@haskell.org Subject: [Haskell-beginners] I have a -> f a. How to get m a -> f m a ? Message-ID: <20170904150634.230dd5ff@Pavel> Content-Type: text/plain; charset=US-ASCII

Hello List!

I have function a -> IO a. How to get function:

Maybe a -> IO (Maybe a) ?

I found in Haskell mails archive such thing:

class IFunctor f where imap :: Idiom i => (s -> i t) -> f s -> i (f t)

which looks similar, but I didn't find any helpfull instances of `IFunctor` class in its package (and unfortunately I don't know what are the indexed types: IMonad, IFunctor, etc). Sure, there is the primitive solution like:

myfunc :: a -> IO a ... f x = case x of Nothing -> return Nothing Just x' -> Just <$> myfunc x'

but more interesting is to know more standard and Haskelish solution (like lift's, etc). May be I miss something very obvious..

=== Best regards, Paul

------------------------------

Message: 2 Date: Mon, 4 Sep 2017 15:14:42 +0300 From: Imants Cekusins To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell Subject: Re: [Haskell-beginners] I have a -> f a. How to get m a -> f m a ? Message-ID: Content-Type: text/plain; charset="utf-8"

...

I have function a -> IO a. How to get function: Maybe a -> IO (Maybe a) ?

Will mapM work:

http://hackage.haskell.org/package/base-4.10.0.0/docs/ Data-Traversable.html#v:mapM

?

On 4 September 2017 at 15:06, Baa wrote:

...

Hello List!

I have function a -> IO a. How to get function:

Maybe a -> IO (Maybe a) ?

I found in Haskell mails archive such thing:

class IFunctor f where imap :: Idiom i => (s -> i t) -> f s -> i (f t)

which looks similar, but I didn't find any helpfull instances of `IFunctor` class in its package (and unfortunately I don't know what are the indexed types: IMonad, IFunctor, etc). Sure, there is the primitive solution like:

myfunc :: a -> IO a ... f x = case x of Nothing -> return Nothing Just x' -> Just <$> myfunc x'

but more interesting is to know more standard and Haskelish solution (like lift's, etc). May be I miss something very obvious..

=== Best regards, Paul _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners