From f389f2ba584ec9443ae3c138fefbd66ea3d99d9a Mon Sep 17 00:00:00 2001
From: Michael Abbott <me@escbook>
Date: Sat, 19 Dec 2020 16:56:19 +0100
Subject: [PATCH] friendlier error message for gradient failure

---
 src/grad/reverse.jl | 2 +-
 src/grad/tracker.jl | 2 +-
 src/grad/zygote.jl  | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/grad/reverse.jl b/src/grad/reverse.jl
index da8b335..9bddd93 100644
--- a/src/grad/reverse.jl
+++ b/src/grad/reverse.jl
@@ -8,7 +8,7 @@ using .ReverseDiff
 ReverseDiff.@grad function (ev::Eval)(args...)
     Z = ev.fwd(ReverseDiff.value.(args)...)
     Z, Δ -> begin
-        isnothing(ev.rev) && error("no gradient definition here!")
+        ev.rev===nothing && throw("No gradient definition found! Running `@tullio` with keyword `verbose=true` may print the reason")
         ev.rev(ReverseDiff.value(Δ), Z, ReverseDiff.value.(args)...)
     end
 end
diff --git a/src/grad/tracker.jl b/src/grad/tracker.jl
index 53e1618..723914f 100644
--- a/src/grad/tracker.jl
+++ b/src/grad/tracker.jl
@@ -8,7 +8,7 @@ using .Tracker
 Tracker.@grad function (ev::Eval)(args...)
     Z = ev.fwd(Tracker.data.(args)...)
     Z, Δ -> begin
-        isnothing(ev.rev) && error("no gradient definition here!")
+        ev.rev===nothing && throw("No gradient definition found! Running `@tullio` with keyword `verbose=true` may print the reason")
         tuple(ev.rev(Tracker.data(Δ), Z, Tracker.data.(args)...)...)
     end
 end
diff --git a/src/grad/zygote.jl b/src/grad/zygote.jl
index 82808f9..f719c79 100644
--- a/src/grad/zygote.jl
+++ b/src/grad/zygote.jl
@@ -4,7 +4,7 @@ using .Zygote
 Zygote.@adjoint function (ev::Eval)(args...)
     Z = ev.fwd(args...)
     Z, Δ -> begin
-        isnothing(ev.rev) && error("no gradient definition here!")
+        ev.rev===nothing && throw("No gradient definition found! Running `@tullio` with keyword `verbose=true` may print the reason")
         tuple(nothing, ev.rev(Δ, Z, args...)...)
     end
 end