Question

This question is about the Data.Vector package.

Given the fact that I’ll never use the old value of a certain cell once the cell is updated. Will the update operation always create a new vector, reflecting the update, or will it be done as an in-place update ?

Note: I know about Data.Vector.Mutable

Answer 1

No, but something even better can happen.

Data.Vector is built using “stream fusion”. This means that if the sequence of operations that you are performing to build up and then tear down the vector can be fused away, then the Vector itself will never even be constructed and your code will turn into an optimized loop.

Fusion works by turning code that would build vectors into code that builds up and tears down streams and then puts the streams into a form that the compiler can see to perform optimizations.

So code that looks like

foo :: Int
foo = sum as
   where as, bs, cs, ds, es :: Vector Int
         as = map (*100) bs 
         bs = take 10 cs
         cs = zipWith (+) (generate 1000 id) ds
         ds = cons 1 $ cons 3 $ map (+2) es 
         es = replicate 24000 0

despite appearing to build up and tear down quite a few very large vectors can fuse all the way down to an inner loop that only calculates and adds 10 numbers.

Doing what you proposed is tricky, because it requires that you know that no references to a term exist anywhere else, which imposes a cost on any attempt to copy a reference into an environment. Moreover, it interacts rather poorly with laziness. You need to attach little affine addenda to the thunk you conspicuously didn’t evaluate yet. But to do this in a multithreaded environment is scarily race prone and hard to get right.