Segfault in Derivative

[jpfairbanks]

This package looks so cool. I was trying to use it based on the readme and came across this error.

It is probably user error, but why does this segfault?

# This causes a seg fault.
using ModelingToolkit

# Define some variables
@IVar t
@DVar x(t) y(t) z(t)
@Param σ ρ β

f(x,y,z) = z*x+y
Derivative(f, x, 1)

   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.0.2 (2018-11-08)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia> include("test/derivatives_fail.jl")
Segmentation fault: 11

[HarrisonGrodin]

This seems like a combination of user and internal error. Hopefully the internal error is solved in #84.

To get the desired behavior (which I'm assuming is differentiating f(x,y,z) with respect to x?), you should be able to use the following:

@Deriv D'~x

expand_derivatives(D(f(x,y,z)))

Derivative doesn't taken into account the chain rule. (In fact, it acts more as a database of partial derivative rules.) Thus, we instead construct a Differential object with respect to x using @Deriv D'~x, apply it to the desired expression, and (symbolically) expand the derivatives.

[ChrisRackauckas]

Yes, the Differential should be used, and Derivative is a behind the scenes way that it's actually implemented in the IR. We need to better separate the internals vs externals of the documentation here.