[Haskell-cafe] order of assertEqual arguments is counterintuitive

[David Feuer]

I think newtypes are awkward here. Better to use a record to get "named
arguments":

data ExpAct a = ExpAct
  { expected :: a
  , actual :: a
  }

assertSame
  :: HasCallStack
  => (Eq a, Show a)
  => String
  -> ExpAct a
  -> Assertion
assertSame s ExpAct{expected=e, actual=a} = assertEqual s e a

Now you can write things like

assertSame "whatever" ExpAct{expected=4, actual=x}

On Tue, Mar 1, 2022, 2:38 PM Viktor Dukhovni <ietf-dane at dukhovni.org> wrote:

> On Tue, Mar 01, 2022 at 12:56:02PM -0500, Daneel Yaitskov wrote:
>
> > I noticed, that I pay more attention than I should, when working with
> > assertions, because I am not sure about argument order i.e. whether the
> > expected value goes first or vice-versa. Does anybody have similar
> thought?
>
> The documentation is quite clear:
>
>
> https://hackage.haskell.org/package/HUnit-1.6.2.0/docs/Test-HUnit-Base.html#v:assertEqual
>
> > Such subtle detail should be easy to grasp with regular practice, but I
> > observe difficulties and I suspect that there is a logical reason for
> that.
> >
> > I spell "assertEqual" expression as:  "Assert that x equals to y"
> > "y" sounds like a model value (i.e. expected value).
>
> As a combinator that can be curried,
>
>     -- Check variable given against fixed wanted
>     assertEqual prefix wanted :: (Eq a, Show a) => a -> Assertion
>
> is I think more useful than:
>
>     -- Check variable wanted against fixed given.
>     assertEqual prefix given :: (Eq a, Show a) => a -> Assertion
>
> > Similar issue with test fixing - I always have to check first, that an
> > expected value is actually one. There is no type safety preventing mixing
> > arguments.
>
> The test works either way of course, the only thing that changes is the
> error message on failure.  You could implement a type-safe wrapper:
>
>     newtype Given a  = Given a
>     newtype Wanted a = Wanted a
>
>     checkEq :: (Eq a, Show a) => String -> Given a -> Wanted a -> Assertion
>     checkEq prefix (Given x) (Wanted y) = assertEqual prefix y x
>
> Then always call via:
>
>     checkEq "Oops" (Given (2 + 2)) (Wanted 5)
>
> --
>     Viktor.
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