Postfix conditional operator in F#

Question

In Perl language one can write something like

someFunction() if $x == 0

i.e. apply condition in postfix notation.

I was sure there must be similar type of expression in F#, since it is so flexible in working with functions. But when I try to write

someFunction() if x = 0

or

someFunction() << if x = 0

I recieve an expected error message. Is there any way to implement more or less general postfix conditional operator in F#?

Answer 1

There is no built-in F# support for this and so I would recommend just using the normal prefix style conditionals (or pattern matching). There is also no standard operator/function that would be common in the community, so the idiomatic style would just use ordinary if.

I think that the best you can do is something like this:

/// Evaluates lazy value only if the specified boolean holds
let assuming b (v:Lazy<_>) = if b then v.Value

// Print foo only when x > 40
let foo() = printfn "hi"
let x = 42
lazy foo() |> assuming (x > 40)

Note that I had to add lazy to the expression (to make sure it is not actually evaluated when the condition does not holds). This works, but it is certainly uglier than writing if x>40 then foo() - but it is a fun thing to experiment with :-)