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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)
----------------------------------------------------------------------
Message: 1
Date: Mon, 4 Sep 2017 15:06:34 +0300
From: Baa <aqua...@gmail.com>
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 <$> myfunc x'
but more interesting is to know more standard and Haskelish solution
(like lift's, etc). May be I miss something very obvious..
===
Best regards, Paul
------------------------------
Message: 2
Date: Mon, 4 Sep 2017 15:14:42 +0300
From: Imants Cekusins <ima...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] I have a -> f a. How to get m a -> f
m a ?
Message-ID:
<cap1qinaehe6hqvs0o9-cfv9vc2x8zvfq_srfxqgvh-yhapz...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
> I have function a -> IO a. How to get function:
> Maybe a -> IO (Maybe a) ?
Will mapM work:
http://hackage.haskell.org/package/base-4.10.0.0/docs/Data-Traversable.html#v:mapM
?
On 4 September 2017 at 15:06, Baa <aqua...@gmail.com> wrote:
> Hello List!
>
> I have function a -> IO a. How to get function:
>
> Maybe a -> IO (Maybe a) ?
>
> I found in Haskell mails archive such thing:
>
> class IFunctor f where
> imap :: Idiom i => (s -> i t) -> f s -> i (f t)
>
> which looks similar, but I didn't find any helpfull instances of
> `IFunctor` class in its package (and unfortunately I don't know what are
> the indexed types: IMonad, IFunctor, etc). Sure, there is the primitive
> solution like:
>
> myfunc :: a -> IO a
> ...
> f x = case x of Nothing -> return Nothing
> Just x' -> Just <$> 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@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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