# What functions might break ForwardDiff.jl?

**URL:** <https://discourse.julialang.org/t/what-functions-might-break-forwarddiff-jl/64694>\
**Category:** General Usage\
**Tags:** forwarddiff\
**Created:** [July 15, 2021, 2:30pm UTC](https://discourse.julialang.org/t/what-functions-might-break-forwarddiff-jl/64694 "2021-07-15T14:30:40Z")\
**Posts on this page:** 1\
**Showing post:** 8

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**Author:** ![bvrb](https://avatars.discourse-cdn.com/v4/letter/b/76d3ee/32.png) [@bvrb](https://discourse.julialang.org/u/bvrb)\
**Post date:** [July 15, 2021, 5:57pm UTC](https://discourse.julialang.org/t/what-functions-might-break-forwarddiff-jl/64694/8 "2021-07-15T17:57:42Z")

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I am not sure whether this is relevant in your case, but I recently ran into the following problem: When the input is an integer, and somewhere down the line there is a conversion to float, it will be a `Float16` instead of a `Float64` (reported here: [`ForwardDiff.derivative(float, 1)` returns `Float16(1.0)` · Issue #362 · JuliaDiff/ForwardDiff.jl · GitHub](https://github.com/JuliaDiff/ForwardDiff.jl/issues/362)).

Compare:

```julia
julia> using ForwardDiff

julia> ForwardDiff.derivative(float, 1)
Float16(1.0)

julia> ForwardDiff.derivative(float, 1.0)
1.0

```

I ran into this when trying to write my own convenience wrapper for the derivative of a complex function via the Jacobian based on a [suggestion on this discourse](https://discourse.julialang.org/t/automatic-differentiation-of-complex-valued-functions/30263/3), and then wondering why seemingly simpler test inputs like `1 + 1im` were giving much less accurate results than those involving e.g. `π`.

The simplest “real-world” case I found where this becomes a problem was when using the function `sincos` (notice the problem won’t appear if using the normal `sin` function):

```julia
using ForwardDiff

julia> mysin(x) = sincos(x)[1]
mysin (generic function with 1 method)

julia> ForwardDiff.derivative(mysin, 1)
Float16(0.5405)

julia> ForwardDiff.derivative(mysin, 1.0)
0.5403023058681398

```

My workaround for the time being was then to just simply convert every input to `Float64` beforehand.

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