2009/1/15 Peter Verswyvelen
When I first read about active patterns in F#, I found it really cool idea, since it allows creating fake data constructors that can be used for pattern matching, giving many views to a single piece of data, and allowing backwards compatibility when you completely change or hide a data structure. So for example one could define a Polar pattern and a Rect pattern that give different views of a Complex number, e.g (pseudo code follows)
pattern Polar c = (mag c, phase c) pattern Rect c = (real c, imag c)
This seems handy:
polarMul (Polar m1 p1) (Polar m2 p2) = mkComplexFromPolar (m1*m2) (p1+p2)
However, I think it is flawed, since the following
case c of Polar _ _ -> "it's polar!" Rect _ _ -> "it's rect!"
seems like valid code but does not make any sense.
I think it's okay, given that we understand the meanings involved. To me it makes about as much sense as this; case c of x -> "it's x!" y -> "it's y!" Which is just wrong code. Maybe the capital letters on Polar and Rect are the confusing bit? Luke