configureTransformer - Cannot invoke with an argument list about siesta

[pcantrell]

If you expect the response content to already have been transformed into a String by the earlier transformers in the pipeline, then do this:

service().configureTransformer("/foo") { $0.content as String }       // example 0

This doesn’t transform the content, but it does cause Siesta to report it as an error if the content comes out as anything other than a string.

If you are getting something else from the transformers, an Int for example, and you want to convert it into a string, do this:

service().configureTransformer("/foo") { String($0.content as Int) }  // example 1

Or if you prefer a very explicit approach:

service().configureTransformer("/foo") {                              // example 2
    (content: Int, entity: Entity) -> String in
    String(content)
}

Example 2 is what the Swift compiler infers from example 1, all spelled out. Those two are equivalent.

The underlying principle to all of this is that configureTransformer(...) uses Swift’s type inference to determine your transformer’s input and output type. That means that your closure needs to make it clear to the compiler exactly what types it expects, resulting in two common confusions:

1. If you just use a bare $0 in your transformer closure, Swift doesn’t know whether you mean the content argument, or the entire (content,entity) tuple. Usually Swift can infer which one you mean, but because configureTransformer(...)’s first argument is a generic type, there’s not enough information in this case. Saying $0.content removes that ambiguity.
2. You need to make sure that $0.content can only have one possible type in the context where you use it, or the compiler can’t infer its type. There are many String(_:) initializers, so you need to pick one using the as clause. Similarly, you need to make sure your closure has an unambiguous return type.

[shaneneuerburg]

Awesome! That helps immensely.

Works great now! Thanks!

[pcantrell]

Glad it helped!

Please keep the issues coming. It’s really helpful to know how people are using Siesta, and where the stumbling blocks are.

[shaneneuerburg]

👍

[gthrb22]

How to map the model for the response string i am using object mapper and am facing the same issue
MyAPI.configureTransformer("foo") { //
Mapper().map($0)
}
Something like this and i get the error as
Cannot invoke 'configureTransformer' with an argument list of type '(String, (_) -> _)'