Add NarrowTypes

src/TableTransforms.jl

@@ -39,6 +39,7 @@ export
   Coerce,
   Levels,
   OneHot,
+  NarrowTypes,
   Identity,
   Center,
   Scale,

src/transforms.jl

@@ -221,6 +221,7 @@ include("transforms/coalesce.jl")
 include("transforms/coerce.jl")
 include("transforms/levels.jl")
 include("transforms/onehot.jl")
+include("transforms/narrowtypes.jl")
 include("transforms/identity.jl")
 include("transforms/center.jl")
 include("transforms/scale.jl")

src/transforms/narrowtypes.jl

@@ -0,0 +1,18 @@
+# ------------------------------------------------------------------
+# Licensed under the MIT License. See LICENSE in the project root.
+# ------------------------------------------------------------------
+
+"""
+    NarrowTypes()
+
+Converts the element type of columns with generic types to more specific types.
+"""
+struct NarrowTypes <: Colwise end
+
+isrevertible(::Type{NarrowTypes}) = true
+
+colcache(::NarrowTypes, x) = eltype(x)
+
+colapply(::NarrowTypes, x, c) = identity.(x)
+
+colrevert(::NarrowTypes, y, c) = collect(c, y)

test/transforms.jl

@@ -1197,6 +1197,24 @@
     @test_throws AssertionError apply(OneHot("c"), t)
   end
 
+  @testset "NarrowTypes" begin
+    a = Integer[4, 6, 5, 9, 3, 1]
+    b = Number[1, 0, 1, true, false, true]
+    c = Any[5, 6, 7, 1.5, 1.6, 1.7]
+    t = Table(; a, b, c)
+
+    T = NarrowTypes()
+    n, c = apply(T, t)
+    @test eltype(n.a) == Int
+    @test eltype(n.b) == Integer
+    @test eltype(n.c) == Real
+    tₒ = revert(T, n, c)
+    @test eltype(tₒ.a) == Integer
+    @test eltype(tₒ.b) == Number
+    @test eltype(tₒ.c) == Any 
+    @test t == tₒ
+  end
+
   @testset "Identity" begin
     x = rand(4000)
     y = rand(4000)