[Haskell-cafe] ANN: LeanCheck v0.9.0 -- enumerative property testing

[Johannes Waldmann]

Yes, leancheck is awesome.


> When should I reach to enumerative testing?

I think enumerative testing
is best suited to algebraic data types.

https://mail.haskell.org/pipermail/haskell-cafe/2018-September/129951.html


In the past I had one standard example
where random enumeration seems better than enumerative:
(non)associativity of floating point ops
(I like to use this in teaching)


That's immediate in Quickcheck

quickCheck $ \ x y z -> (x::Double) + (y+z) == (x+y)+z
*** Failed! Falsifiable (after 3 tests and 3 shrinks):
-0.3
-1.4266097805446862
-1.2543117889178947


You don't get to see a counterexample in Smallcheck

Prelude Test.SmallCheck> smallCheck 5 $ \ x y z -> (x::Double) + (y+z)
== (x+y)+z
Completed 50653 tests without failure.


To my surprise, we now have

Prelude Test.LeanCheck> check $ \ x y z -> (x::Double) + (y+z) == (x+y)+z
*** Failed! Falsifiable (after 87 tests):
0.0 Infinity (-Infinity)


and I get the above (with "real" numbers) via

Prelude Test.LeanCheck> checkFor 1000 $ \ x y z -> isInfinite x ||
isInfinite y || isInfinite z || (x::Double) + (y+z) == (x+y)+z
*** Failed! Falsifiable (after 306 tests):
1.0 1.0 0.3333333333333333


Turns out the enumeration in Leancheck uses Rationals,
while Smallcheck uses  encodeFloat, which happens to
produce only "nice" numbers (very few bits set) in the beginning.

Prelude Test.LeanCheck> list 3 series :: [Double]
[0.0,1.0,-1.0,2.0,0.5,-2.0,4.0,0.25,-0.5,-4.0,-0.25]

Prelude Test.LeanCheck> take 12 $ list :: [Double]
[0.0,1.0,-1.0,Infinity,0.5,2.0,-Infinity,-0.5,-2.0,0.3333333333333333,3.0,-0.3333333333333333]


- J.W.


PS: this post wouldn't be complete without me complaining
that I cannot (easily) put the type annotation
where it belongs -  in the declaration of the name:

Prelude Test.LeanCheck> check $ \ (x::Double) y z -> x + (y+z) == (x+y)+z

<interactive>:22:12: error:
    Illegal type signature: ‘Double’
      Type signatures are only allowed in patterns with ScopedTypeVariables

Wat? TypeVariables? There aren't any!