[Haskell-beginners] Get rid of Maybes in complex types

[Imants Cekusins]

how about a function (or a Monad Transformer) that checks values in one
place:

check::A Maybe -> Maybe (A Identity)

after values were checked, the after-checked functions will deal with A
Identity

the end result would be
   Maybe out



On 6 July 2017 at 13:01, Baa <aquagnu at gmail.com> wrote:

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