CODEWITHSUNDEEP
ocaml
glagola
glagola

Reputation: 2232

Strange behaviour of default parameter value

I just start learning OCaml. And I have a question, why this code outputs different results every time I call him?

let b () = Unix.time ();;
let a ?(c = b ()) () = c;;
a ();;
...
a ();;

I expected, that default value of c will be compute once.

Upvotes: 4

Views: 65

Answers (1)

Jeffrey Scofield
Jeffrey Scofield

Reputation: 66803

Optional parameters are described in Section 6.7 of the OCaml manual. Here's what it says:

A function of the form

 fun ? lab :(  pattern =  expr0 ) ->  expr

is equivalent to

fun ? lab : ident ->
    let pattern = match ident with
   | Some ident -> ident
   | None -> expr0
in
expr

where ident is a fresh variable, except that it is unspecified when expr0 is evaluated.

This shows expr0 being evaluated at each call, if the optional parameter is not supplied. I.e., expr0 is inside the lambda, not outside.

Upvotes: 4

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