Merge pull request #118 from JuliaDiff/ox/howthunkdoc

Move some stuff about using thunks to the docs

Author: oxinabox

docs/src/writing_good_rules.md

@@ -26,6 +26,17 @@ return @thunk(∂A)
 ```
 In the bad example `foo(x)` gets computed eagerly, and all that the thunk is doing is wrapping the already calculated result in a function that returns it.
 
+Do not use `@thunk` if this would be equal or more work than actually evaluating the expression itself.
+Examples being:
+- The expression being a constant
+- The expression is merely wrapping something in a `struct`, such as `Adjoint(x)` or `Diagonal(x)`
+- The expression being itself a `thunk`
+- The expression being from another `rrule` or `frule`;
+  it would be `@thunk`ed if required by the defining rule already.
+- There is only one derivative being returned, so from the fact that the user called
+  `frule`/`rrule` they clearly will want to use that one.
+
+
 ## Be careful with using `adjoint` when you mean `transpose`
 
 Remember for complex numbers `a'` (i.e. `adjoint(a)`) takes the complex conjugate.

src/differentials/thunks.jl

@@ -57,14 +57,8 @@ Also if we did `Zero() * res[1]` then the result would be `Zero()` and `f(x)` wo
 with a field for each variable used in the expression, and call overloaded.
 
 Do not use `@thunk` if this would be equal or more work than actually evaluating the expression itself.
-Examples being:
-- The expression being a constant
-- The expression is merely wrapping something in a `struct`, such as `Adjoint(x)` or `Diagonal(x)`
-- The expression being itself a `thunk`
-- The expression being from another `rrule` or `frule`;
-  it would be `@thunk`ed if required by the defining rule already.
-- There is only one derivative being returned, so from the fact that the user called
-  `frule`/`rrule` they clearly will want to use that one.
+
+For more details see the manual section [on using thunks effectively](http://www.juliadiff.org/ChainRulesCore.jl/dev/writing_good_rules.html#Use-Thunks-appropriately-1)
 """
 struct Thunk{F} <: AbstractThunk
     f::F