performance - Why is it so slow to create records with a field that references a big value in F#?

Question

In the code below that is executed as an .fsx script, the final line takes around 30 seconds to finish. I assumed that since records are reference types, the final line only creates records with a field that references an (immutable) large value, and so it should be very fast. Why is it slow and how can I fix it?

type BigType = { Big: Set<int> }
type CollectionType = { BigVal: BigType; SmallVal: int }

let b = { Big = set [ 0 .. 999999 ] }
let mySet = set [ 0 .. 50 ]

#time

mySet |> Set.map (fun x -> { BigVal = b; SmallVal = x })

Thank you.

Answer 1

One thing to notice here is that the order you define the fields in type CollectionType = { BigVal: BigType; SmallVal: int } makes a difference. If you try:

type BigType = { Big: Set<int> }
type CollectionType = { SmallVal: int; BigVal: BigType; }
let b = { Big = set [ 0 .. 999999 ] }
let mySet = set [ 0 .. 50 ]
#time
mySet |> Set.map (fun x -> { BigVal = b; SmallVal = x })

Instead the time taken goes from Real: 00:00:34.385 to Real: 00:00:00.002.

NB: I was originally concerned that this behaviour could not be relied on and might change without warning; however as nasosev has found this behaviour is described in the F# language specification see section 8.15.3 of version 4.1 of the document.