WIP+RFC add chunked mode

src/config.jl

@@ -89,3 +89,19 @@ See also: [`frule_via_ad`](@ref), [`RuleConfig`](@ref) and the documentation on
 [rule configurations and calling back into AD](@ref config)
 """
 function rrule_via_ad end
+
+abstract type ChunkedRuleCapability end
+"""
+HasChunkedMode
+
+This trait indicates that a `RuleConfig{>:HasChunkedMode}` can perform chunked AD.
+"""
+struct HasChunkedMode <: ChunkedRuleCapability end
+
+"""
+NoChunkedMode
+
+This is the complement to [`HasChunkedMode`](@ref). To avoid ambiguities [`RuleConfig`]s
+that do not support chunked AD should be `RuleConfig{>:NoChunkedMode}`.
+"""
+struct NoChunkedMode <: ChunkedRuleCapability end

src/rules.jl

@@ -78,6 +78,32 @@ function (::typeof(frule_kwfunc))(kws::Any, ::typeof(frule), ::RuleConfig, args.
     return frule_kwfunc(kws, frule, args...)
 end
 
+struct ProductTangent{P}
+    partials::P
+end
+
+function frule(::RuleConfig{>:HasChunkedMode}, (Δf, Δx...), f, args...)
+    return frule((Δf, Δx...), f, args...)
+end
+
+function frule(::RuleConfig{>:HasChunkedMode}, (Δf, Δx)::Tuple{Any,ProductTangent}, f, args...)
+    fx = frule((Δf, first(Δx)), args...)[1]
+    dfx = ProductTangent(map(Δrow->frule((Δf, Δrow), f, args...)[2], Δx))
+    return (fx, dfx)
+end
+
+function rrule(::RuleConfig{>:HasChunkedMode}, f, args...)
+    y, back = rrule(args...)
+    return y, ApplyBack(back)
+end
+
+struct ApplyBack{F}
+    back::F
+end
+
+(a::ApplyBack)(dy) = a.back(dy)
+(a::ApplyBack)(dy::ProductTangent) = ProductTangent(map(a.back, dy.partials))  # or some Tangent recursion?
+
 """
     rrule([::RuleConfig,] f, x...)
 
@@ -149,7 +175,7 @@ const NO_RRULE_DOC = """
 This is an piece of infastructure supporting opting out of [`rrule`](@ref).
 It follows the signature for `rrule` exactly.
 A collection of type-tuples is stored in its method-table.
-If something has this defined, it means that it must having a must also have a `rrule`, 
+If something has this defined, it means that it must having a must also have a `rrule`,
 defined that returns `nothing`.
 
 !!! warning "Do not overload no_rrule directly"

src/tangent_types/thunks.jl

@@ -135,8 +135,9 @@ Define a [`Thunk`](@ref) wrapping the `expr`, to lazily defer its evaluation.
 macro thunk(body)
     # Basically `:(Thunk(() -> $(esc(body))))` but use the location where it is defined.
     # so we get useful stack traces if it errors.
-    func = Expr(:->, Expr(:tuple), Expr(:block, __source__, body))
-    return :(Thunk($(esc(func))))
+    #func = Expr(:->, Expr(:tuple), Expr(:block, __source__, body))
+    #return :(Thunk($(esc(func))))
+    return esc(body)
 end
 
 """