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From Matthias Boehm <>
Subject Re: UDFs Within Expressions
Date Wed, 29 Mar 2017 23:18:43 GMT
Well, this would indeed be a very useful extension - I've actually seen
many use cases, where new users ran into issues with simple expressions
like X[i,i] = foo(). In the general case, the problem with UDFs is that
they can have - in contrast to builtin functions - multiple returns. These
multiple returns would translate to HOPs with multiple outputs, which in
turn cannot be represented with our current HOP DAG representation because
a HOP represents an operation and the characteristics of a single output,
but potentially with many consumers. This is also the reason why builtin
functions with multiple outputs (i.e., lu, eigen, qr) are internally mapped
to FunctionOps with the same restrictions.

However, we could actually allow UDFs with a single output in expressions.
This would require a generalization of how results variables are bound but
should not take too much effort. Additional it would require a full pass
through the compiler to remove any assumptions that FunctionOps always
appear as DAG root nodes. Bottom line: We could realistically add it to our
feature list for the 1.0 release.


On Wed, Mar 29, 2017 at 3:55 PM, <> wrote:

> Currently, it is not possible to use UDFs within an expression.  I.e. I'd
> like to be able to use something like `out = (-1/2) *
> util::my_function(x)`.  This would of course extend to more elaborate
> expressions.  Also, note that we *are* able to use built-in functions
> within expressions.
> I think it would be good to allow for this.  Are there any issues that
> would make this difficult?
> -Mike
> --
> Mike Dusenberry
> GitHub:
> LinkedIn:
> Sent from my iPhone.

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