Re: Beginners Digest, Vol 111, Issue 4

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Re: Beginners Digest, Vol 111, Issue 4

Jack Brandt
Would a specialization of return work, as in 

f = return :: Maybe a -> IO (Maybe a)?

On Mon, Sep 4, 2017 at 7:00 AM, <[hidden email]> wrote:
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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)


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Message: 1
Date: Mon, 4 Sep 2017 15:06:34 +0300
From: Baa <[hidden email]>
To: [hidden email]
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 <[hidden email]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
        beginner-level topics related to Haskell <[hidden email]>
Subject: Re: [Haskell-beginners] I have a -> f a. How to get m a -> f
        m a ?
Message-ID:
        <[hidden email]>
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 <[hidden email]> 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
> [hidden email]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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