is there a way to get this to work ```fun lt Result gt doit

Question

is there a way to get this to work ```fun lt Result gt doit lambda gt Result= Unit ``` when the lambda is not passed i want Result to be of type Unit I get the error Required Result found Unit

christophsturm · Answer

currently i do it without default parameter and have a seconf function that has no lambda parameter and calls the first function with ` `

ephemient · Answer

how would you expect a `doit lt NotUnit gt ` call to work

christophsturm · Answer

this is just a reduced example maybe its probably too reduced slightly smiling face

christophsturm · Answer

``` suspend fun lt ContextDependency gt context name String given suspend gt ContextDependency contextLambda suspend ContextDSL lt ContextDependency gt gt Unit ``` this is how it originally looks

ephemient · Answer

which lambda are you trying to default

christophsturm · Answer

the given lambda

christophsturm · Answer

the usual case is no given is given and so its unit ContextDependency is Unit

ephemient · Answer

yeah I can t imagine that working then there s always going to be a way of writing `context name inferred context of ContextDSL lt something other than Unit gt `

christophsturm · Answer

ok but shouldn t the default paramter be basically the same then the overloaded function that I m now using ``` override suspend fun context name String function ContextLambda context name function ```

ephemient · Answer

no because it s specialized

ephemient · Answer

when you write a `fun lt T gt foo ` the caller can choose any `T` up to constraints you declare on it you can t constrain it differently based on whether a parameter is present or defaulted

christophsturm · Answer

ok so the default parameter can not force the generic type into a type

ephemient · Answer

no

christophsturm · Answer

do you mean its impossible because the kotlin compiler does not support it or because it conceptually wrong

ephemient · Answer

<https youtrack jetbrains com issue KT 52519 Allow the default argument in a function to dictate the type if no argument is supplied> currently the compiler does not support it but I also don t see a good plan for how to actually change the language to enable it

Giorgos Makris · Answer

There is a way to do it You would basically create a sealed interface which will be implemented by different data classes each data class will have 1 field the function you want to use Basically functions as algebraic data type stuck out tongue The downside is that you will have to pattern match the parameter to the correct function

Giorgos Makris · Answer

Your default will also be explicit

ephemient · Answer

I consider that isomorphic to the just write an additional overload solution since you re writing two implementations one forwarding to the other

christophsturm · Answer

if by isomorphic you mean that one solution is super simple and the other one very complex then i agree slightly smiling face

ephemient · Answer

by isomorphic I mean that they have the same shape

christophsturm · Answer

ok but isnt that just a fancy way of saying equivalent

ephemient · Answer

it literally means same shape doesn t have to be equivalent in complexity to be isomorphic

christophsturm · Answer

I m just trying to understand when solution a is isomorphic to solution b is a helpful statement and what decisions it would help someone make

ephemient · Answer

it helps in saying that the two solutions do not differ in this regard and your decision should be guided by other concerns