f# - Completely lost in trying to mutate property in sequence -

i @ loss why code doesn't mutate member variable in sequence of types:

for p in prescrs     p.atc <- "a"     c in p.drug.components         s in c.substances                 s.dosetotal.adjust <- adjustkg                 s.dosetotal.time <- "day"                 s.dosetotal.unit <- s.drugconcentration.unit                 s.doserate.adjust <- adjustkg                 s.doserate.time <- "day"                 s.doserate.unit <- s.drugconcentration.unit 

prescrs sequence of prescriptions simple 'poco' defined type member values. don't have clue why doesn't work.

i tried simple test case like:

type itertest () =     member val name = "" get, set     member val itertests = [] |> list.toseq : itertest seq get, set  let iterseq =      [         new itertest(name = "test1")         new itertest(name = "test2")     ]      |> list.toseq  iterseq |> seq.iter(fun x -> x.itertests <- iterseq) iterseq |> seq.iter(fun x ->      x.itertests     |> seq.iter(fun x' -> x'.name <- "itered")) 

but here result expected. so, can't quite reproduce problem???

found solution (without understanding problem above). when first convert prescrs sequence list like:

let prescrs = prescrs |> seq.tolist 

and imperative looping, properties mutated.

try sample:

type mutable() =      member val iterated = false get, set  let muts = seq.init 5 (fun _ -> printfn "init"; mutable())  let muts2 = muts // try again let muts2 = muts |> list.ofseq  printfn "before iter"  in muts2     printfn "iter"         a.iterated <- true   printfn "after iter"   muts2 |> list.ofseq 

and check how iter , init interleaved.

seqs lazy, not cached once computed. if imperatively try mutate of elements in prescrs sequence, goes away once pull prescrs again. if change prescrs concrete collection type list before doing mutation, no longer hit same problem. note things might trickier if have seq inside seq inside seq.

the best idea avoid mutation in first place though.