Add isinvertible trait

Author: juliohm

src/identity.jl

@@ -11,6 +11,10 @@ struct Identity <: Transform end
 
 isrevertible(::Type{Identity}) = true
 
+isinvertible(::Type{Identity}) = true
+
+Base.inv(::Identity) = Identity()
+
 apply(::Identity, object) = object, nothing
 
 revert(::Identity, newobject, cache) = newobject

src/interface.jl

@@ -30,9 +30,27 @@ function assertions end
 
 Tells whether or not the `transform` is revertible, i.e. supports a
 [`revert`](@ref) function. Defaults to `false` for new transform types.
+
+Transforms can be revertible and yet don't be invertible. Invertibility
+is a mathematical concept, whereas revertibility is a computational concept.
+
+See also [`isinvertible`](@ref).
 """
 function isrevertible end
 
+"""
+    isinvertible(transform)
+
+Tells whether or not the `transform` is invertible, i.e. supports a
+[`inv`](@ref) function. Defaults to `false` for new transform types.
+
+Transforms can be invertible in the mathematical sense, i.e., there
+exists a one-to-one mapping between input and output spaces.
+
+See also [`isrevertible`](@ref).
+"""
+function isinvertible end
+
 """
     prep = preprocess(transform, object)
 
@@ -79,6 +97,10 @@ isrevertible(transform::Transform) =
   isrevertible(typeof(transform))
 isrevertible(::Type{<:Transform}) = false
 
+isinvertible(transform::Transform) =
+  isinvertible(typeof(transform))
+isinvertible(::Type{<:Transform}) = false
+
 preprocess(transform::Transform, object) = nothing
 
 (transform::Transform)(object) =

src/sequential.jl

@@ -25,6 +25,8 @@ end
 
 isrevertible(s::SequentialTransform) = all(isrevertible, s.transforms)
 
+isinvertible(s::SequentialTransform) = all(isinvertible, s.transforms)
+
 function apply(s::SequentialTransform, table)
   allcache = []
   current  = table

test/runtests.jl

@@ -2,5 +2,8 @@ using TransformsBase
 using Test
 
 @testset "TransformsBase.jl" begin
+  @test TransformsBase.isrevertible(Identity())
+  @test TransformsBase.isinvertible(Identity())
+  @test inv(Identity()) == Identity()
   @test (Identity() → Identity()) == Identity()
 end