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<p>Hi Olaf,</p>
<p>You might like quickcheck-higherorder, "A QuickCheck extension
for properties of higher-order values."</p>
<p><a class="moz-txt-link-freetext" href="https://hackage.haskell.org/package/quickcheck-higherorder">https://hackage.haskell.org/package/quickcheck-higherorder</a><br>
</p>
<p>One of the key bits is a class for testable equality, which may
have an instance for (a -> b), unlike Eq:<br>
</p>
<pre><code class="language-haskell">class TestEq a where
(=?) :: a -> a -> Property
</code><code class="language-haskell"></code></pre>
<p>The package has more bells and whistles to further streamline
writing properties that quantify over functions.</p>
<p>If you only ever compare unary first-order functions, you really
only need the single instance TestEq (a -> b), which you can
extract as a self-contained operator:</p>
<p>(=?) :: (Coarbitrary a, Show a, Arbitrary b, Eq b, Show b) =>
(a -> b) -> (a -> b) -> Property<br>
(=?) f g = property $ \x -> f x === g x<br>
</p>
Regards,<br>
Li-yao<br>
<pre><code class="language-haskell"></code></pre>
<p></p>
<div class="moz-cite-prefix">On 2023-05-31 12:25 PM, Olaf Klinke
wrote:<br>
</div>
<div class="replaced-blockquote"
cite="mid:0626487deb20283260693c580cc8871a684d530b.camel@aatal-apotheke.de"
type="cite">
<pre class="moz-quote-pre" wrap="">Dear Cafe,
The expression
\x -> f x == g x
is a testable property, as long as values for x can be randomly
generated. For clarity I'd prefer a point-free style, e.g.
f ≡ g
Are there extensions to QuickCheck that let me write this? The
QuickCheck package itself does not seem to contain such an operator. My
current work-around is a
newtype ExtensionalEquality a b
that holds two functions of type (a -> b) and a Testable instance for
it. But I've got a hunch that I re-invented some wheel here. (My
ExtensionalEquality is isomorphic to
Refl (a -> b) (a -> b)
but Refl ist conceptually about type equality, not term equality.)
Thanks
Olaf
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