Applies a finite partial function, with a backup function for undefined points.
DESCRIPTION
This is one of a suite of operations on finite partial functions, type
('a,'b)func. These may sometimes be preferable to ordinary functions since
they permit more operations such as equality comparison, extraction of domain
etc. If f is a finite partial function, g a conventional function and x
an argument, tryapply f g x tries to apply f to x as with apply f x,
but instead returns g x is f is undefined on x.
FAILURE CONDITIONS
Can only fail if the backup function fails.
EXAMPLE
# applyd undefined (fun x -> x) 1;;
val it : int = 1
# applyd (1 |=> 2) (fun x -> x) 1;;
val it : int = 2