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

6 Jul 2017


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

Imants Cekusins