tweak

Author: ablaom

src/traits.jl

@@ -54,9 +54,12 @@ named_properties = NamedTuple{properties}(getproperty.(Ref(algorithm), propertie
 @assert algorithm == LearnAPI.constructor(algorithm)(; named_properties...)
 ```
 
+which can be tested with `@assert LearnAPI.clone(algorithm) == algorithm`.
+
 The keyword constructor provided by `LearnAPI.constructor` must provide default values for
 all properties, with the exception of those that can take other LearnAPI.jl algorithms as
-values.
+values. These can be provided with the default `nothing`, with the constructor throwing an
+error if the default value persists.
 
 """
 function constructor end

test/integration/iterative_algorithms.jl

@@ -36,6 +36,12 @@ Instantiate a bagged ensemble of `n` regressors, with base regressor `atom`, etc
 Ensemble(atom; rng=Random.default_rng(), n=10) =
     Ensemble(atom, rng, n) # `LearnAPI.constructor` defined later
 
+# pure keyword argument constructor:
+function Ensemble(; atom=nothing, kwargs...)
+    isnothing(atom) && error("You must specify `atom=...` ")
+    Ensemble(atom; kwargs...)
+end
+
 struct EnsembleFitted
     algorithm::Ensemble
     atom::Ridge