Equality inference in F# + records with mutable fields

Question

The common equality/comparison members design guideline is to not implement structural equality on mutable reference types, but take a look at F# record types with mutable fields:

type Value = { mutable value: int }

let mutableRecord = { value = 1 }
let xs = Map.ofList [ mutableRecord, "abc"
                      { value = 2 }, "def" ]

let abc = Map.find { value=1 } xs
mutableRecord.value <- 3
let abc = Map.find { value=3 } xs // KeyNotFoundException!

The Map is sorted internally, but mutable record fields allows me to change ordering while record instance is already inside map and this is very bad.

I think F# should infer [<NoEquality>] and [<NoComparison>] modes for F# record types that declares mutable fields, isn't it?

score 5 · Accepted Answer · answered Jan 18 '11 at 17:08

That's not an unreasonable stance.

There might be some clever ways to leverage this feature usefully, though I haven't thought about it deeply enough. This is basically the same thing as when you put a mutable type in a Dictionary, and you get what you deserve. (Languages can't prevent every misuse, which is why we have design guidelines and programmer judgment to fill in the gaps :) )

Anyway, there's no changing it now.

Equality inference in F# + records with mutable fields

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