<div dir="ltr">how about a function (or a Monad Transformer) that checks values in one place: <div><br></div><div>check::A Maybe -> Maybe (A Identity)</div><div><br></div><div>after values were checked, the after-checked functions will deal with A Identity </div><div><br></div><div>the end result would be </div><div> Maybe out</div><div><br></div><div><br></div></div><div class="gmail_extra"><br><div class="gmail_quote">On 6 July 2017 at 13:01, Baa <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">But will it work if I switch from one monad to another? Actually, I<br>
have something like piping/conduit, and if I switch items in pipe from<br>
`A Maybe` to `A Idenitity` - will it work? Whether it will be compiled?<br>
<br>
Although I certainly can "map" items from one type to another...<br>
Idea looks interesting sure :)<br>
<div class="HOEnZb"><div class="h5"><br>
<br>
> Identity<br>
><br>
> <a href="http://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Identity.html" rel="noreferrer" target="_blank">http://hackage.haskell.org/<wbr>package/mtl-2.2.1/docs/<wbr>Control-Monad-Identity.html</a><br>
><br>
> may work:<br>
><br>
> data A m = A {<br>
> a1 :: m B<br>
> }<br>
> data B m = B {<br>
> b1 :: m C<br>
> ... }<br>
><br>
> m: Maybe or Identity<br>
><br>
> - any good?<br>
><br>
><br>
><br>
> On 6 July 2017 at 11:12, Baa <<a href="mailto:aquagnu@gmail.com">aquagnu@gmail.com</a>> wrote:<br>
><br>
> > Hello Dear List!<br>
> ><br>
> > Consider, I retrieve from external source some data. Internally it's<br>
> > represented as some complex type with `Maybe` fields, even more,<br>
> > some of fields are record types and have `Maybe` fields too. They<br>
> > are Maybe's because some information in this data can be missing<br>
> > (user error or it not very valuable and can be skipped):<br>
> ><br>
> > data A = A {<br>
> > a1 :: Maybe B<br>
> > ... }<br>
> > data B = B {<br>
> > b1 :: Maybe C<br>
> > ... }<br>
> ><br>
> > I retrieve it from network, files, i.e. external world, then I<br>
> > validate it, report errors of some missing fields, fix another one<br>
> > (which can be fixed, for example, replace Nothing with `Just<br>
> > default_value` or even I can fix `Just wrong` to `Just right`, etc,<br>
> > etc). After all of this, I know that I have "clean" data, so all my<br>
> > complex types now have `Just right_value` fields. But I need to<br>
> > process them as optional, with possible Nothing case! To avoid it I<br>
> > must create copies of `A`, `B`, etc, where `a1`, `b1` will be `B`,<br>
> > `C`, not `Maybe B`, `Maybe C`. Sure, it's not a case.<br>
> ><br>
> > After processing and filtering, I create, for example, some<br>
> > resulting objects:<br>
> ><br>
> > data Result {<br>
> > a :: A -- not Maybe!<br>
> > ... }<br>
> ><br>
> > And even more: `a::A` in `Result` (I know it, after filtering) will<br>
> > not contain Nothings, only `Just right_values`s.<br>
> ><br>
> > But each function which consumes `A` must do something with possible<br>
> > Nothing values even after filtering and fixing of `A`s.<br>
> ><br>
> > I have, for example, function:<br>
> ><br>
> > createResults :: [A] -> [Result]<br>
> > createResults alst =<br>
> > ...<br>
> > case of (a1 theA) -><br>
> > Just right_value -> ...<br>
> > Nothing -><br>
> > logError<br>
> > undefined -- can not happen<br>
> ><br>
> > Fun here is: that it happens (I found bug in my filtering<br>
> > code with this `undefined`). But now I thought about it: what is the<br>
> > idiomatic way to solve such situation? When you need to have:<br>
> ><br>
> > - COMPLEX type WITH Maybes<br>
> > - the same type WITHOUT Maybes<br>
> ><br>
> > Alternative is to keep this Maybes to the very end of processing,<br>
> > what I don't like. Or to have types copies, which is more terrible,<br>
> > sure.<br>
> ><br>
> > PS. I threw IOs away to show only the crux of the problem.<br>
> ><br>
> > ---<br>
> > Cheers,<br>
> > Paul<br>
> > ______________________________<wbr>_________________<br>
> > Beginners mailing list<br>
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> ><br>
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</div></div></blockquote></div><br></div>