Entering partial derivatives?

[Han]

None of those work because your syntax is ambiguous. There is no way (from a software point of view) to tell if the substitution should be applied prior to executing the command diff() or after. While we all understand your intended meaning, such is the nature of the where function.

As for syntax, it is usually:

expression|x=value

or

expression|{x=value,y=value,…}

It appears that the where function gives precedence to substitution over evaluation of functions. On the other hand, subst() has reverse precedence.

Edit:

diff(x^2*y,x)|{x=r,y=s} produces 2*r*s

subst(diff(x^2*y),{x=1,y=2}) produces 4