<div dir="ltr"><div><span style="font-size:12.8px">> </span><span style="font-size:12.8px">Why.. </span></div><div><span style="font-size:12.8px"><br></span></div><div><span style="font-size:12.8px">what would </span></div><div><span style="font-size:12.8px"> check::A Maybe -> A Identity</span><br></div><div><span style="font-size:12.8px"><br></span></div><div><span style="font-size:12.8px">do if some A field were Nothing? fill in a default value?</span></div></div><div class="gmail_extra"><br><div class="gmail_quote">On 6 July 2017 at 14:13, 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">Imants, I'm not sure that I understood signatures.<br>
<br>
Why<br>
<span class=""> check::A Maybe -> Maybe (A Identity)<br>
<br>
</span>but not<br>
check::A Maybe -> A Identity<br>
<br>
?<br>
<br>
To filter (Nothing items must be throw out)? No more reasons for it?<br>
<div class="HOEnZb"><div class="h5"><br>
<br>
> how about a function (or a Monad Transformer) that checks values in<br>
> one place:<br>
><br>
> check::A Maybe -> Maybe (A Identity)<br>
><br>
> after values were checked, the after-checked functions will deal with<br>
> A Identity<br>
><br>
> the end result would be<br>
> Maybe out<br>
><br>
><br>
><br>
> On 6 July 2017 at 13:01, Baa <<a href="mailto:aquagnu@gmail.com">aquagnu@gmail.com</a>> wrote:<br>
><br>
> > 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<br>
> > from `A Maybe` to `A Idenitity` - will it work? Whether it will be<br>
> > compiled?<br>
> ><br>
> > Although I certainly can "map" items from one type to another...<br>
> > Idea looks interesting sure :)<br>
> ><br>
> ><br>
> > > Identity<br>
> > ><br>
> > > <a href="http://hackage.haskell.org/package/mtl-2.2.1/docs/" rel="noreferrer" target="_blank">http://hackage.haskell.org/<wbr>package/mtl-2.2.1/docs/</a><br>
> > Control-Monad-Identity.html<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<br>
> > > > it's represented as some complex type with `Maybe` fields, even<br>
> > > > more, some of fields are record types and have `Maybe` fields<br>
> > > > too. They are Maybe's because some information in this data can<br>
> > > > be missing (user error or it not very valuable and can be<br>
> > > > 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<br>
> > > > one (which can be fixed, for example, replace Nothing with `Just<br>
> > > > default_value` or even I can fix `Just wrong` to `Just right`,<br>
> > > > etc, etc). After all of this, I know that I have "clean" data,<br>
> > > > so all my complex types now have `Just right_value` fields. But<br>
> > > > I need to process them as optional, with possible Nothing case!<br>
> > > > To avoid it I must create copies of `A`, `B`, etc, where `a1`,<br>
> > > > `b1` will be `B`, `C`, not `Maybe B`, `Maybe C`. Sure, it's not<br>
> > > > 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)<br>
> > > > will not contain Nothings, only `Just right_values`s.<br>
> > > ><br>
> > > > But each function which consumes `A` must do something with<br>
> > > > possible 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<br>
> > > > is the idiomatic way to solve such situation? When you need to<br>
> > > > 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<br>
> > > > processing, what I don't like. Or to have types copies, which<br>
> > > > is more terrible, sure.<br>
> > > ><br>
> > > > PS. I threw IOs away to show only the crux of the problem.<br>
> > > ><br>
> > > > ---<br>
> > > > Cheers,<br>
> > > > Paul<br>
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</div></div></blockquote></div><br></div>