In your abstract example, instead of:
andThen2 : (Var2 -> a) -> Type2 -> Maybe a
you should try to write:
getVar3 : Type2 -> Maybe Var3
With that approach, the pattern Christian suggested would look like this
for your scribbles example:
model.study.status
|> getProgressingSteps
|> Maybe.map Pivot.getCurrent
|> Maybe.andThen getScribbles
|> Maybe.map (Scribbles.update scribblesMsg model)
|> Maybe.withDefault model
where you simply have to write
getProgressingSteps : Study.Status -> Maybe (Pivot Study.Step)
getScribbles : Study.Step -> Maybe Scribbles
On Wed, Mar 29, 2017 at 9:45 PM, Matthieu Pizenberg <
[email protected]> wrote:
> We have found that this pattern is typically a good case for chaining a
>> bunch of andThen statements with a withDefault statement at the end
>> together. The one thing is you would have to be able to adjust your values
>> to all be Maybes.
>>
>
> Yes Christian, I am wondering if people are aware of some more generic
> continuation pattern matching syntax. The andThen expression is very useful
> for monadic types like Maybe. (sorry for the M term use ^^ but it seemed
> appropiate even if I'm not into category theory). But in my exemples, I'm
> using Union types that are not Maybes. I guess what I could do is create my
> own "andThenX" like this:
>
> f1 : Var1 -> Type2
> f2 : Var2 -> Type3
> ...
>
> Type Type2
> = DataType2 Var2
> | ...
>
> Type Type3
> = DataType3 Var3
> | ...
>
> andThen2 : (Var2 -> a) -> Type2 -> Maybe a
> andThen2 f2 type2 =
> case type2 of
> DataType2 var2 ->
> Just (f2 var2)
> _ ->
> Nothing
>
> andThen3 : (Var3 -> a) -> Type3 -> Maybe a
> andThen3 f3 type3 =
> case type3 of
> DataType3 var3 ->
> Just (f3 var3)
> _ ->
> Nothing
> ...
>
> And then (no pun intended ^^) do something like:
>
> f1 var1
> |> andThen2 f2
> |> andThen (andThen3 f3)
> |> ...
> |> andThen (andThenN fN)
> |> withDefault otherwise
>
> This has the advantage of having a clear flow, but the inconvenient that I
> need to write the my special andThenX functions.
> What do you think of this approach?
>
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