immutability - Is using a StringBuilder a right thing to do in F#? -

stringbuiler mutable object, f# encourages employing immutability as possible. 1 should use transformation rather mutation. apply stringbuilder when comes building string in f#? there f# immutable alternative it? if so, alternative efficient?

a snippet

i think using stringbuilder in f# fine - fact sb.append returns current instance of stringbuilder means can used fold function. though still imperative (the object mutated), fits reasonably functional style when not expose references stringbuilder.

but equally, can construct list of strings , concatenate them using string.concat - efficient using stringbuilder (it slower, not - , faster concatenating strings using +)

so, lists give similar performance, immutable (and work concurrency etc.) - fit if building string algorithmically, because can append strings front of list - efficient operation on lists (and reverse string). also, using list expressions gives convenient syntax:

// concatenating strings using + (2.3 seconds) let s1 = [ in 0 .. 25000 -> "hello " ] |> seq.reduce (+) s1.length  // creating immutable list , using string.concat (5 ms) let s2 = [ in 0 .. 25000 -> "hello " ] |> string.concat "" s2.length  // creating lazy sequence , concatenating using stringbuilder & fold (5 ms) let s3 =    seq { in 0 .. 25000 -> "hello " }   |> seq.fold(fun (sb:system.text.stringbuilder) s ->        sb.append(s)) (new system.text.stringbuilder())   |> fun x -> x.tostring() s3.length  // imperative solution using stringbuilder , loop (1 ms) let s4 =    ( let sb = new system.text.stringbuilder()     in 0 .. 25000 sb.append("hello ") |> ignore     sb.tostring() ) s4.length 

the times measured on my, fast, work machine using #time in f# interactive - quite faster in release build, think representative.