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

6 Jul 2017


      Unfortunately, no.
...
is it possible to replace Nothing with defaults when A, B are
init'ed ?
On 6 July 2017 at 14:33, Baa  wrote:
...
Hmm, yes. `Nothing` becomes `Identity default_value`. Respectively
`A Maybe` becomes `A Identity`. I'm not sure will it work but it
looks alluringly ;)
...
...
Why..
what would
    check::A Maybe -> A Identity
do if some A field were Nothing?  fill in a default value?
On 6 July 2017 at 14:13, Baa  wrote:
...
Imants, I'm not sure that I understood signatures.
Why
  check::A Maybe -> Maybe (A Identity)
but not
  check::A Maybe -> A Identity
?
To filter (Nothing items must be throw out)? No more reasons
for it?
...
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  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  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@haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> >
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