relax restrictions on model type in resampling

src/resampling.jl

@@ -31,10 +31,6 @@ const ERR_INVALID_OPERATION = ArgumentError(
 _ambiguous_operation(model, measure) =
     "`$measure` does not support a `model` with "*
     "`prediction_type(model) == :$(prediction_type(model))`. "
-err_ambiguous_operation(model, measure) = ArgumentError(
-    _ambiguous_operation(model, measure)*
-    "\nUnable to infer an appropriate operation for `$measure`. "*
-    "Explicitly specify `operation=...` or `operations=...`. ")
 err_incompatible_prediction_types(model, measure) = ArgumentError(
     _ambiguous_operation(model, measure)*
     "If your model is truly making probabilistic predictions, try explicitly "*
@@ -65,11 +61,25 @@ ERR_MEASURES_DETERMINISTIC(measure) = ArgumentError(
         "and so is not supported by `$measure`. "*LOG_AVOID
 )
 
-# ==================================================================
-## MODEL TYPES THAT CAN BE EVALUATED
+err_ambiguous_operation(model, measure) = ArgumentError(
+    _ambiguous_operation(model, measure)*
+    "\nUnable to infer an appropriate operation for `$measure`. "*
+    "Explicitly specify `operation=...` or `operations=...`. "*
+    "Possible value(s) are: $PREDICT_OPERATIONS_STRING. "
+)
+
+ERR_UNSUPPORTED_PREDICTION_TYPE = ArgumentError(
+    """
+
+    The `prediction_type` of your model needs to be one of: `:deterministic`,
+    `:probabilistic`, or `:interval`. Does your model implement one of these operations:
+    $PREDICT_OPERATIONS_STRING? If so, you can try explicitly specifying `operation=...`
+    or `operations=...` (and consider posting an issue to have the model review it's
+    definition of `MLJModelInterface.prediction_type`). Otherwise, performance
+    evaluation is not supported.
 
-# not exported:
-const Measurable = Union{Supervised, Annotator}
+   """
+)
 
 # ==================================================================
 ## RESAMPLING STRATEGIES
@@ -987,7 +997,7 @@ function _actual_operations(operation::Nothing,
                 throw(err_ambiguous_operation(model, m))
             end
         else
-            throw(err_ambiguous_operation(model, m))
+            throw(ERR_UNSUPPORTED_PREDICTION_TYPE)
         end
     end
 end
@@ -1137,7 +1147,7 @@ See also [`evaluate`](@ref), [`PerformanceEvaluation`](@ref),
 
 """
 function evaluate!(
-    mach::Machine{<:Measurable};
+    mach::Machine;
     resampling=CV(),
     measures=nothing,
     measure=measures,
@@ -1235,7 +1245,7 @@ Returns a  [`PerformanceEvaluation`](@ref) object.
 See also [`evaluate!`](@ref).
 
 """
-evaluate(model::Measurable, args...; cache=true, kwargs...) =
+evaluate(model::Model, args...; cache=true, kwargs...) =
     evaluate!(machine(model, args...; cache=cache); kwargs...)
 
 # -------------------------------------------------------------------

test/resampling.jl

@@ -25,6 +25,8 @@ end
 struct DummyInterval <: Interval end
 dummy_interval=DummyInterval()
 
+struct GoofyTransformer <: Unsupervised end
+
 dummy_measure_det(yhat, y) = 42
 API.@trait(
     typeof(dummy_measure_det),
@@ -115,6 +117,12 @@ API.@trait(
         MLJBase.err_ambiguous_operation(dummy_interval, LogLoss()),
         MLJBase._actual_operations(nothing,
                                    [LogLoss(), ], dummy_interval, 1))
+
+    # model not have a valid `prediction_type`:
+    @test_throws(
+        MLJBase.ERR_UNSUPPORTED_PREDICTION_TYPE,
+        MLJBase._actual_operations(nothing, [LogLoss(),], GoofyTransformer(), 0),
+    )
 end
 
 @everywhere begin
@@ -935,7 +943,23 @@ end
     end
 end
 
-# DUMMY LOGGER
+
+# # TRANSFORMER WITH PREDICT
+
+struct PredictingTransformer <:Unsupervised end
+MLJBase.fit(::PredictingTransformer, verbosity, X, y) = (mean(y), nothing, nothing)
+MLJBase.predict(::PredictingTransformer, fitresult, X) = fill(fitresult, nrows(X))
+MLJBase.prediction_type(::Type{<:PredictingTransformer}) = :deterministic
+
+@testset "`Unsupervised` model with a predict" begin
+    X = rand(10)
+    y = fill(42.0, 10)
+    e = evaluate(PredictingTransformer(), X, y, resampling=Holdout(), measure=l2)
+    @test e.measurement[1] ≈ 0
+end
+
+
+# # DUMMY LOGGER
 
 struct  DummyLogger end