Make BandMatrixRow callable

Author: sjavis

src/MatrixFields/band_matrix_row.jl

@@ -157,3 +157,11 @@ inv(::BandMatrixRow{ld, bw}) where {ld, bw} = error(
     "The inverse of a matrix with $bw diagonals is generally a dense matrix, \
      so it cannot be represented using BandMatrixRows",
 )
+
+@generated function (row::BandMatrixRow{ld, bw, T})(args::Vararg{Any, N}) where {ld, bw, T, N}
+    N == bw || error(
+        "BandMatrixRow with bandwidth $bw expected $bw arguments, but got $N"
+    )
+    terms = [:(row.entries[$i] * args[$i]) for i in 1:bw]
+    return Expr(:call, :+, terms...)
+end

test/MatrixFields/band_matrix_row.jl

@@ -58,4 +58,11 @@ include("matrix_field_test_utils.jl")
     @test zero(typeof(DiagonalMatrixRow(1))) == DiagonalMatrixRow(0)
     @test eltype(typeof(DiagonalMatrixRow(1))) == Int
     @test inv(DiagonalMatrixRow(1)) == DiagonalMatrixRow(float(1))
+
+    # Test callable behaviour
+    @test DiagonalMatrixRow(2.0)(10.0) == 20.0
+    @test BidiagonalMatrixRow(1.0, 2.0)(10.0, 5.0) == 20.0
+    @test TridiagonalMatrixRow(-1.0, 2.0, -1.0)(10.0, 25.0, 12.0) == 28.0
+    @test_throws ErrorException TridiagonalMatrixRow(1, 2, 3)(10.0, 5.0)
+    @test_throws ErrorException TridiagonalMatrixRow(1, 2, 3)(10.0, 5.0, 1.0, 2.0)
 end