[Haskell-beginners] 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, <beginners-request at haskell.org> wrote:

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> Today's Topics:
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>    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 <aquagnu at gmail.com>
> To: beginners at haskell.org
> Subject: [Haskell-beginners] I have a -> f a. How to get m a -> f m a
>         ?
> Message-ID: <20170904150634.230dd5ff at 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 <$> myfunc x'
>
> but more interesting is to know more standard and Haskelish solution
> (like lift's, etc). May be I miss something very obvious..
>
> ===
> Best regards, Paul
>
>
> ------------------------------
>
> Message: 2
> Date: Mon, 4 Sep 2017 15:14:42 +0300
> From: Imants Cekusins <imantc at gmail.com>
> To: The Haskell-Beginners Mailing List - Discussion of primarily
>         beginner-level topics related to Haskell <beginners at haskell.org>
> Subject: Re: [Haskell-beginners] I have a -> f a. How to get m a -> f
>         m a ?
> Message-ID:
>         <CAP1qinaEhE6HqvS0O9-CfV9Vc2x8ZvfQ_SRFXQgVH-
> yhapZZtg at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> > I have function a -> IO a. How to get function:
> >  Maybe a -> IO (Maybe a)  ?
>
> Will mapM work:
>
> http://hackage.haskell.org/package/base-4.10.0.0/docs/
> Data-Traversable.html#v:mapM
>
> ?
>
>
>
> On 4 September 2017 at 15:06, Baa <aquagnu at gmail.com> wrote:
>
> > Hello List!
> >
> > I have function a -> IO a. How to get function:
> >
> >   Maybe a -> IO (Maybe a)  ?
> >
> > I found in Haskell mails archive such thing:
> >
> >    class IFunctor f where
> >      imap :: Idiom i => (s -> i t) -> f s -> i (f t)
> >
> > which looks similar, but I didn't find any helpfull instances of
> > `IFunctor` class in its package (and unfortunately I don't know what are
> > the indexed types: IMonad, IFunctor, etc). Sure, there is the primitive
> > solution like:
> >
> >    myfunc :: a -> IO a
> >    ...
> >    f x = case x of Nothing -> return Nothing
> >                    Just x' -> Just <$> 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 at haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> >
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> End of Beginners Digest, Vol 111, Issue 4
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