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

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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