How to make f# query expressions reusable?

Question

i am trying to get accustomed with f# query expressions.

First of all, they are definitely eager, because:

let arr = [1; 2; 3]
let q = query { for item in arr do select item; count };;

val arr : int list = [1; 2; 3]
val q : int = 3

I want to reuse that query, so I try lazy computation:

let arr = ref [1; 2; 3]
let q = lazy query { for item in !arr do select item; count }
q.Value;;

val arr : int list ref = {contents = [1; 2; 3];}
val q : Lazy<int> = 3
val it : int = 3

Now, i want to re-evalute the query:

arr := [1; 2; 3; 4]
q.Force();;

val it : int = 3

So, nothing happened, it wouldn't recalculate. How can I use the query several times without redefining it?

A bit strange for me is that there's no tool to re-evaluate an expression — Rustam, Apr 03 '14 at 13:27
There is a tool and it's called a function, as mention in the answer by @Tomas Petricek. Functions can be treated as parametrized expressions. — Eugene Fotin, Apr 03 '14 at 14:40
That's right, I got focused on `lazy` keyword and tryed to use it inappropriately. — Rustam, Apr 04 '14 at 06:02

score 7 · Accepted Answer · answered Apr 03 '14 at 13:00

7

It is hard to say what the best approach is based on your very minimal example, but the simplest way to write reusable code is to write a function:

let q (arr:seq<'T>) = 
  query { for item in arr do select item; count };;

And then you can reuse it just by calling it:

> q [1;2;3];;
val it : int = 3
> q [1;2;3;4];;
val it : int = 4

answered Apr 03 '14 at 13:00

Tomas Petricek

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Why didn't i think of that? – Rustam Apr 03 '14 at 13:03

How to make f# query expressions reusable?

1 Answers1