<div dir="ltr"><span style="font-size:14.4px;text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">> alternative does not apply function to first Nothing</span><br><div><span style="font-size:14.4px;text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><br></span></div><div><span style="font-size:14.4px;text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><|> or other functions from Alternative may not be the entire function to solve Marc's problem.</span></div><div><span style="font-size:14.4px;text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><br></span></div><div><span style="font-size:14.4px;text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">It is a possibly useful function that is a bit tricky to find. ;)</span></div><div><span style="font-size:14.4px;text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><br></span></div></div><div class="gmail_extra"><br><div class="gmail_quote">On 31 July 2018 at 11:26, Paul <span dir="ltr"><<a href="mailto:aquagnu@gmail.com" target="_blank">aquagnu@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
<p>But alternative does not apply function to first Nothing. `maybe`
can exec function on Nothing or apply another one on Just.</p>
<p>maybe (g x) id (f x) ?<br>
</p>
<br>
<div class="m_3513032222926179820moz-cite-prefix">31.07.2018 11:10, Imants Cekusins
wrotes:<br>
</div><div><div class="h5">
<blockquote type="cite">
<div dir="ltr">Salut Marc,
<div><br>
</div>
<div>Are you looking for Alternative by any chance:</div>
<div>
<div style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial"><a href="http://hackage.haskell.org/package/base-4.11.1.0/docs/Control-Applicative.html#t:Alternative" target="_blank">http://hackage.haskell.org/<wbr>package/base-4.11.1.0/docs/<wbr>Control-Applicative.html#t:<wbr>Alternative</a><br>
</div>
<br class="m_3513032222926179820gmail-Apple-interchange-newline">
<br>
</div>
<div>
<div>Prelude> :m Control.Applicative</div>
<div>Prelude Control.Applicative> Just 1 <|> Just
2::Maybe Int</div>
<div>Just 1</div>
<div>Prelude Control.Applicative> Nothing <|> Just
2::Maybe Int</div>
<div>Just 2</div>
<div><br>
</div>
<div><br>
</div>
<div><br>
</div>
</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On 31 July 2018 at 10:47, Marc Busqué <span dir="ltr"><<a href="mailto:marc@lamarciana.com" target="_blank">marc@lamarciana.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Apologies,
I clearly asked the question in a rushed way...<br>
<br>
I'm working with [req](<a href="http://hackage.haskell.org/package/req" rel="noreferrer" target="_blank">http://hackage.haskell.o<wbr>rg/package/req</a>)
package.<br>
I need to parse a url from a string, but I don't know
whether its schema<br>
will be `http` or `https`. However, `req` just provides me
with two<br>
functions:<br>
<br>
```<br>
parseUrlHttp :: ByteString -> Maybe (Url Http, Option
scheme)<br>
parseUrlHttps :: ByteString -> Maybe (Url Https, Option
scheme)<br>
```<br>
<br>
As I don't know the schema beforehand, I have to apply one
of the two<br>
functions, do a case analysis on its result and, depending
on the<br>
result, call the second one or return the first result.<br>
<br>
What I think is a common case here (perhaps I'm wrong) is
the need to<br>
choose between two or more `Maybe` values where at most one
of them will<br>
be a `Just`. `maybe` does not do that. `catMaybes` could be
used for<br>
that when all the `Maybe` have the same inner type, but it
is not the<br>
exact abstraction and it would need more code (just like
simply doing a<br>
case analysis).<br>
<br>
Thanks and, again, sorry for the hurried question before (I
don't like<br>
when I see it from others :()<br>
<br>
Marc Busqué<br>
<a href="http://waiting-for-dev.github.io/about/" rel="noreferrer" target="_blank">http://waiting-for-dev.github.<wbr>io/about/</a><br>
<br>
On Tue, 31 Jul 2018, Paul wrote:<br>
<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<br>
Something like "maybe" function?<br>
<br>
<br>
31.07.2018 10:07, Marc Busqué wrotes:<br>
Hi!<br>
<br>
I have two functions:<br>
<br>
```<br>
foo :: a -> Maybe b<br>
bar :: a -> Maybe c<br>
```<br>
<br>
From which I want to build a higher order function:<br>
<br>
```<br>
foobar :: a -> (a -> Maybe b) -> (a ->
Maybe c) -> Either b c<br>
```<br>
<br>
The implementation I need is:<br>
<br>
```<br>
foobar x f g =<br>
case (f x) of<br>
Nothing -> g x<br>
Just y -> Just y<br>
```<br>
<br>
I'm a bit surprised that looking at hoogle I don't
find a built-in<br>
solution for this quite common need for `Maybe`
types (or perhaps for<br>
any monad).<br>
<br>
Am I looking in the wrong way? Does it exist a
similar abstraction but<br>
with a different shape?<br>
<br>
Thanks in advance,<br>
<br>
Marc Busqué<br>
<a href="http://waiting-for-dev.github.io/about/" rel="noreferrer" target="_blank">http://waiting-for-dev.github.<wbr>io/about/</a><br>
<br>
<br>
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