Inside `@diff`, shouldn't `similar(Result{Array})` return `Result{Array}`?

[NHDaly]

Inside `@diff`, shouldn't `similar(Result{Array})` return `Result{Array}`?

[denizyuret]

The Result type represents the result of an operation that is recorded for gradient calculation. If a function has zero gradient (i.e. no dependence on the input), it does not need to get recorded, therefore it does not create a Result. similar is an example, so are size, length, eltype etc.

Ideally this should all be transparent to the programmer. The only functions in the API open to the programmer are Param, @diff, value and grad (i.e. I should be able to change the name or fields of Result without breaking any of your code).

Type stability is a difficult topic: while running outside of a @diff context, all Params are ignored and no Results are generated eliminating most of the overhead of AutoGrad. This means the same variable could be Array during some calls and Result{Array} during others.

In your case if you want to prevent the type change inside the for loop it may be possible to do term = 0 * coef[:,ndim] in the beginning.