add catch for missing target in resampling

src/resampling.jl

@@ -68,7 +68,7 @@ err_ambiguous_operation(model, measure) = ArgumentError(
     "Possible value(s) are: $PREDICT_OPERATIONS_STRING. "
 )
 
-ERR_UNSUPPORTED_PREDICTION_TYPE = ArgumentError(
+const ERR_UNSUPPORTED_PREDICTION_TYPE = ArgumentError(
     """
 
     The `prediction_type` of your model needs to be one of: `:deterministic`,
@@ -81,6 +81,18 @@ ERR_UNSUPPORTED_PREDICTION_TYPE = ArgumentError(
    """
 )
 
+const ERR_NEED_TARGET = ArgumentError(
+   """
+
+    To evaluate a model's performance you must provide a target variable `y`, as in
+    `evaluate(model, X, y; options...)` or
+
+        mach = machine(model, X, y)
+        evaluate!(mach; options...)
+
+    """
+)
+
 # ==================================================================
 ## RESAMPLING STRATEGIES
 
@@ -1170,6 +1182,8 @@ function evaluate!(
     # weights, measures, operations, and dispatches a
     # strategy-specific `evaluate!`
 
+    length(mach.args) > 1 || throw(ERR_NEED_TARGET)
+
     repeats > 0 || error("Need `repeats > 0`. ")
 
     if resampling isa TrainTestPairs

test/resampling.jl

@@ -948,14 +948,20 @@ end
 
 struct PredictingTransformer <:Unsupervised end
 MLJBase.fit(::PredictingTransformer, verbosity, X, y) = (mean(y), nothing, nothing)
+MLJBase.fit(::PredictingTransformer, verbosity, X) = (nothing, nothing, nothing)
 MLJBase.predict(::PredictingTransformer, fitresult, X) = fill(fitresult, nrows(X))
+MLJBase.predict(::PredictingTransformer, ::Nothing, X) = nothing
 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
+    @test_throws(
+        MLJBase.ERR_NEED_TARGET,
+        evaluate(PredictingTransformer(), X, measure=l2),
+    )
 end