Prop.filter (==>) executes property even if condition does not hold

[isberg]

I ran into a confusing situation regarding the ==> operator (which uses Prop.filter: Prop.filter condition assertion). I was expecting the assertion should not be executed if the condition was false, but code like this will blow up (when the list is empty):

open FsCheck
open FsCheck.FSharp
open FsCheck.Xunit

[<Property>]
let ``take the head, then put it back`` (xs : int list) : Property = 
    (List.length xs > 0) ==> 
        (List.head xs :: List.tail xs = xs)

I get it that functional code is a lot safer when pure, but considering that assertion is stated to be 'Testable, which if I understand it correctly means either bool or unit, then it seems to be expected that the assertion can have side-effects.

Am I missing something, is there a more appropriate way to this?

[kurtschelfthout]

Due to F#’s eagerness, the right hand side is always executed. There is no practical way I know of FsCheck can avoid this without user intervention. You can wrap the right hand side in Lazy to avoid this.

[kurtschelfthout]

See the docs here for more https://fscheck.github.io/FsCheck/Properties.html#Lazy-Properties

[isberg]

Thanks @kurtschelfthout! That was exactly what I needed to know. :-)

The following code show what I really was trying to achieve: getting FsCheck and Unquote to be friends.

open FsCheck
open FsCheck.FSharp
open Swensen.Unquote

let lazy_test : Quotations.Expr<bool> -> Lazy<unit> = fun expr -> lazy test expr

type ListProperties =
  static member ``explicit lazy`` (xs : int list) : Property =
    (List.length xs > 0) ==>
        lazy test <@ List.head xs::List.tail xs = xs @>
        
  static member ``implizit lazy`` (xs : int list) : Property =
    (List.length xs > 0) ==>
        lazy_test <@ List.head xs::List.tail xs = xs @>

Check.QuickAll<ListProperties> ()

[isberg]

This issue relates to issue #130 and above code might be relevant for that issue also.