On 01/01/2013 03:35 PM, Matthias Felleisen wrote:
Neil,
thanks for the user story. We should hear more of those for all kinds of
corners in our world.
You're welcome!
Question: did you start the math library in an untyped form (and switch) or did
you go with types from the get-go?
It's all been in Typed Racket from the beginning. This was initially
because I mostly had floating-point functions planned, and I wanted TR's
help proving that I never used non-flonums by mistake. Also, I wanted
the special functions to have performance competitive with C
implementations.
When I decided to do arrays, Typed Racket was again the natural choice.
First, because it's not possible to use `require/typed' to import
polymorphic struct types. Second, because it allowed me to avoid a lot
of runtime checks.
I discovered reasons #3 and #4 while I was writing `math/array':
3. Passing around higher-order functions can get confusing. I can't
speak for anyone else, but when I get something like `array-axis-reduce'
- which is like a fold but with complicated types - to pass the type
checker, my code is usually correct.
4. TR's types for Racket's numeric tower, especially the additional
`Index' type, makes it easy to safely use unsafe indexing. Throughout
`math/array', values of type `Index' are always either a size (e.g. a
vector length) or an index that has been proved to be less than a size.
IOW, if I can get TR to prove j : Index, I'm satisfied that
(unsafe-vector-ref xs j) is actually safe. Also, if `j' is a loop
counter, TR can usually compare and increment using unsafe fixnum ops.
There are a bunch of other little things that are like #4 (which is only
a big thing because indexes are so prevalent in `math/array'). Maybe
I'll collect those into a separate user story.
Neil ⊥
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