Nice to see the developments for this package. I have not had an opportunity yet to use it in earnest, but I do have an application in mind. One thing, though, not criticism, just a remark: if you use it to evaluate an array, then all variables should be arrays, right? No possibilty to combine a scalar with an array? Something along these lines:
real :: a(100)
real :: b
...
expression = 'exp( -b * a)'
...
write(*,*) f%evaluate( x )