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Add NotImplemented and NotImplementedRule #28

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Add NotImplemented and NotImplementedRule #28

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30 changes: 29 additions & 1 deletion src/differentials.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -46,7 +46,8 @@ This way, we don't need to implement promotion/conversion rules between subtypes
of `AbstractDifferential` to resolve potential ambiguities.
=#

const PRECEDENCE_LIST = [:wirtinger, :casted, :zero, :dne, :one, :thunk, :fallback]
const PRECEDENCE_LIST = [:wirtinger, :casted, :zero, :dne, :notimplemented, :one,
:thunk, :fallback]

global defs = Expr(:block)

Expand Down Expand Up @@ -227,6 +228,33 @@ mul_dne(::DNE, ::DNE) = DNE()
mul_dne(::DNE, ::Any) = DNE()
mul_dne(::Any, ::DNE) = DNE()

#####
##### `NotImplemented`
#####

"""
NotImplemented <: AbstractDifferential

A differential type which behaves similar to [`DNE`](@ref) but instead signifies that
the actual differential is not implemented in ChainRules, not that it does not exist.
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Can there be a "not yet" added here?
Maybe even a "PRs to remove not implemented rules are welcome"

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Good idea.

"""
struct NotImplemented <: AbstractDifferential end

extern(::NotImplemented) = error("`NotImplemented` cannot be converted to an external type.")

Base.Broadcast.broadcastable(::NotImplemented) = Ref(NotImplemented())

Base.iterate(x::NotImplemented) = (x, nothing)
Base.iterate(x::NotImplemented, ::Any) = nothing

add_notimplemented(::NotImplemented, ::NotImplemented) = NotImplemented()
add_notimplemented(::NotImplemented, b) = b
add_notimplemented(a, ::NotImplemented) = a

mul_notimplemented(::NotImplemented, ::NotImplemented) = NotImplemented()
mul_notimplemented(::NotImplemented, ::Any) = NotImplemented()
mul_notimplemented(::Any, ::NotImplemented) = NotImplemented()

#####
##### `One`
#####
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14 changes: 14 additions & 0 deletions src/rules.jl
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Expand Up @@ -158,6 +158,20 @@ struct DNERule <: AbstractRule end

DNERule(args...) = DNE()

#####
##### `NotImplementedRule`
#####

"""
NotImplementedRule <: AbstractRule

Rule indicating that a particular derivative is not implemented by ChainRules.
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As with @oxinabox 's comment above, I would like to see this expanded slightly to say more about what a NotImplementedRule does mean. Some context would be helpful e.g.

"""
Rule indicating that a particular derivative is not yet implemented by ChainRules. Different from [`DNERule`](@ref) in that this rule does not imply non-differentiability, but rather that no one has implemented it yet.
"""

Note that this does not imply nondifferentiability; for that, use [`DNERule`](@ref).
"""
struct NotImplementedRule <: AbstractRule end

NotImplementedRule(args...) = NotImplemented()

#####
##### `WirtingerRule`
#####
Expand Down