Fix input types, improve readability

docs/src/api.md

@@ -88,3 +88,9 @@ kernelpdmat
 nystrom
 NystromFact
 ```
+
+## Optional Utilities
+If the [Kronecker.jl](https://github.com/MichielStock/Kronecker.jl) package is loaded, KernelFunctions also provides the following functions.
+```@docs
+kronecker_kernelmatrix
+```

src/matrix/kernelkroneckermat.jl

@@ -27,31 +27,39 @@ end
 """
 @inline iskroncompatible(κ::Kernel) = false # Default return for kernels
 
-function _kernelmatrix_kroneckerjl_helper(::MOInputIsotopicByFeatures, Kfeatures, Koutputs)
+function _kernelmatrix_kroneckerjl_helper(
+    ::Type{<:MOInputIsotopicByFeatures}, Kfeatures, Koutputs
+)
     return Kronecker.kronecker(Kfeatures, Koutputs)
 end
 
-function _kernelmatrix_kroneckerjl_helper(::MOInputIsotopicByOutputs, Kfeatures, Koutputs)
+function _kernelmatrix_kroneckerjl_helper(
+    ::Type{<:MOInputIsotopicByOutputs}, Kfeatures, Koutputs
+)
     return Kronecker.kronecker(Koutputs, Kfeatures)
 end
 
+"""
+    kronecker_kernelmatrix(k::Union{IndependentMOKernel,IntrinsicCoregionMOKernel}, x::MOI, y::MOI) 
+    where {MOI<:IsotopicMOInputsUnion}
+
+If Kronecker.jl is loaded, computes the `kernelmatrix` for the `IndependentMOKernel` and the `IntrinsicCoregionMOKernel`, but returns a lazy kronecker product. This object can be very efficiently inverted or decomposed. Also refer to `kernelmatrix` documentation. 
+"""
 function kronecker_kernelmatrix(
-    k::Union{IndependentMOKernel,IntrinsicCoregionMOKernel},
-    x::IsotopicMOInputsUnion,
-    y::IsotopicMOInputsUnion,
-)
+    k::Union{IndependentMOKernel,IntrinsicCoregionMOKernel}, x::MOI, y::MOI
+) where {MOI<:IsotopicMOInputsUnion}
     @assert x.out_dim == y.out_dim
     Kfeatures = kernelmatrix(k.kernel, x.x, y.x)
     Koutputs = _mo_output_covariance(k, x.out_dim)
-    return _kernelmatrix_kroneckerjl_helper(x, Kfeatures, Koutputs)
+    return _kernelmatrix_kroneckerjl_helper(MOI, Kfeatures, Koutputs)
 end
 
 function kronecker_kernelmatrix(
-    k::Union{IndependentMOKernel,IntrinsicCoregionMOKernel}, x::IsotopicMOInputsUnion
-)
+    k::Union{IndependentMOKernel,IntrinsicCoregionMOKernel}, x::MOI
+) where {MOI<:IsotopicMOInputsUnion}
     Kfeatures = kernelmatrix(k.kernel, x.x)
     Koutputs = _mo_output_covariance(k, x.out_dim)
-    return _kernelmatrix_kroneckerjl_helper(x, Kfeatures, Koutputs)
+    return _kernelmatrix_kroneckerjl_helper(MOI, Kfeatures, Koutputs)
 end
 
 function kronecker_kernelmatrix(

src/mokernels/independent.jl

@@ -30,25 +30,22 @@ end
 _mo_output_covariance(k::IndependentMOKernel, out_dim) = Eye{Bool}(out_dim)
 
 function kernelmatrix(
-    k::IndependentMOKernel, x::IsotopicMOInputsUnion, y::IsotopicMOInputsUnion
-)
+    k::IndependentMOKernel, x::MOI, y::MOI
+) where {MOI<:IsotopicMOInputsUnion}
     @assert x.out_dim == y.out_dim
     Kfeatures = kernelmatrix(k.kernel, x.x, y.x)
     Koutputs = _mo_output_covariance(k, x.out_dim)
-    return _kernelmatrix_kron_helper(x, Kfeatures, Koutputs)
+    return _kernelmatrix_kron_helper(MOI, Kfeatures, Koutputs)
 end
 
 if VERSION >= v"1.6"
     function kernelmatrix!(
-        K::AbstractMatrix,
-        k::IndependentMOKernel,
-        x::IsotopicMOInputsUnion,
-        y::IsotopicMOInputsUnion,
-    )
+        K::AbstractMatrix, k::IndependentMOKernel, x::MOI, y::MOI
+    ) where {MOI<:IsotopicMOInputsUnion}
         @assert x.out_dim == y.out_dim
         Kfeatures = kernelmatrix(k.kernel, x.x, y.x)
         Koutputs = _mo_output_covariance(k, x.out_dim)
-        return _kernelmatrix_kron_helper!(K, x, Kfeatures, Koutputs)
+        return _kernelmatrix_kron_helper!(K, MOI, Kfeatures, Koutputs)
     end
 end
 

src/mokernels/intrinsiccoregion.jl

@@ -48,25 +48,22 @@ function _mo_output_covariance(k::IntrinsicCoregionMOKernel, out_dim)
 end
 
 function kernelmatrix(
-    k::IntrinsicCoregionMOKernel, x::IsotopicMOInputsUnion, y::IsotopicMOInputsUnion
-)
+    k::IntrinsicCoregionMOKernel, x::MOI, y::MOI
+) where {MOI<:IsotopicMOInputsUnion}
     @assert x.out_dim == y.out_dim
     Kfeatures = kernelmatrix(k.kernel, x.x, y.x)
     Koutputs = _mo_output_covariance(k, x.out_dim)
-    return _kernelmatrix_kron_helper(x, Kfeatures, Koutputs)
+    return _kernelmatrix_kron_helper(MOI, Kfeatures, Koutputs)
 end
 
 if VERSION >= v"1.6"
     function kernelmatrix!(
-        K::AbstractMatrix,
-        k::IntrinsicCoregionMOKernel,
-        x::IsotopicMOInputsUnion,
-        y::IsotopicMOInputsUnion,
-    )
+        K::AbstractMatrix, k::IntrinsicCoregionMOKernel, x::MOI, y::MOI
+    ) where {MOI<:IsotopicMOInputsUnion}
         @assert x.out_dim == y.out_dim
         Kfeatures = kernelmatrix(k.kernel, x.x, y.x)
         Koutputs = _mo_output_covariance(k, x.out_dim)
-        return _kernelmatrix_kron_helper!(K, x, Kfeatures, Koutputs)
+        return _kernelmatrix_kron_helper!(K, MOI, Kfeatures, Koutputs)
     end
 end
 

src/mokernels/mokernel.jl

@@ -5,20 +5,24 @@ Abstract type for kernels with multiple outpus.
 """
 abstract type MOKernel <: Kernel end
 
-function _kernelmatrix_kron_helper(::MOInputIsotopicByFeatures, Kfeatures, Koutputs)
+function _kernelmatrix_kron_helper(::Type{<:MOInputIsotopicByFeatures}, Kfeatures, Koutputs)
     return kron(Kfeatures, Koutputs)
 end
 
-function _kernelmatrix_kron_helper(::MOInputIsotopicByOutputs, Kfeatures, Koutputs)
+function _kernelmatrix_kron_helper(::Type{<:MOInputIsotopicByOutputs}, Kfeatures, Koutputs)
     return kron(Koutputs, Kfeatures)
 end
 
 if VERSION >= v"1.6"
-    function _kernelmatrix_kron_helper!(K, ::MOInputIsotopicByFeatures, Kfeatures, Koutputs)
+    function _kernelmatrix_kron_helper!(
+        K, ::Type{<:MOInputIsotopicByFeatures}, Kfeatures, Koutputs
+    )
         return kron!(K, Kfeatures, Koutputs)
     end
 
-    function _kernelmatrix_kron_helper!(K, ::MOInputIsotopicByOutputs, Kfeatures, Koutputs)
+    function _kernelmatrix_kron_helper!(
+        K, ::Type{<:MOInputIsotopicByOutputs}, Kfeatures, Koutputs
+    )
         return kron!(K, Koutputs, Kfeatures)
     end
 end