Haskell's records are a bit annoying, and type-classes often group together too many methods, which means you make early decisions about future unknown requirements, and IMO you always get it wrong :-)<div><div><br>After having read an email in the cafe about the Noop language & Self language, I realized that what I really would like to have is "strong duck typing" on records (or is it called structural subtyping? or prototype-based-objects? or something like that)<div>
<div><br>For example (silly example full of inaccuracies, but you get the picture):<br><br><font class="Apple-style-span" face="'courier new', monospace">class HasPosition a where<br> position :: a -> Point<br>
withPosition :: Point -> a -> a<br><br>class HasVelocity a where<br> velocity :: a -> Vector<br> withVelocity :: Vector -> a -> a<br></font><br>which we really should write as<br><br><font class="Apple-style-span" face="'courier new', monospace">field HasPosition :: Point<br>
field HasVelocity :: Vector</font><br><br>And then </div><div><br><font class="Apple-style-span" face="'courier new', monospace">record IsKinetic :: HasPosition HasVelocity<br></font><br>suppose we write a function like<br>
<br><font class="Apple-style-span" face="'courier new', monospace">kineticEulerStep dt k = withPosition (position k .+^ dt *^ velocity k) k<br></font><br>kineticEulerStep will work on any type a that HasPosition and HasVelocity, and would get inferred signature<br>
<br><font class="Apple-style-span" face="'courier new', monospace">kineticEulerStep :: IsKinetic a => Float -> a -> a<br></font><br>which is identical to<br><br><font class="Apple-style-span" face="'courier new', monospace">kineticEulerStep :: (HasPosition a, HasVelocity a) => Float -> a -> a<br>
</font><br>So basically kineticEulerStep accepts anything that HasPosition and HasVelocity, whatever it is.<br><br>So if it walks like a duck and ..., then it is a duck, but statically known... <br><br>We could also do<br>
<br><font class="Apple-style-span" face="'courier new', monospace">field HasForce :: Vector<br>field HasMass :: Float<br><br>record IsDynamic :: IsKinetic HasForce HasMass<br><br>acceleration d = force d ^/ mass d<br>
withAcceleration a d = withForce (a ^* mass d) d<br><br>dynamicEulerStep dt d = withVelocity (velocity d ^+^ dt *^ acceleration d)<br></font><br>Of course you would also need type families to be really correct since Vector, Point, etc should also be parametrized.</div>
<div><br></div><div>And really kineticEulerStep might also work on something that HasVelocity and HasAcceleration (since the code in dynamicEulerStep is almost the same as kineticEulerStep), so better abstraction might be needed.</div>
<div><br></div><div><div>I'm not sure what kind of overhead a system like this would have in Haskell, since I suspect the many dictionaries are often not optimized away.</div><div><br></div></div><div>I think for Haskell prime,<a href="http://repetae.net/recent/out/classalias.html"> something like this was suggested</a>, but is was rejected?</div>
<div><div><br>Languages like OCaml and <a href="http://haxe.org/manual/2_types">haXe</a> also provide a similar feature?</div><div><div><br></div><div>I would like to collect ways of doing this in Haskell, without boilerplate, and preferably without runtime overhead.</div>
<div><br></div><div>I remember reading OOHaskell a while time ago, and while I didn't understand a lot of it, I recall it also was doing a similar thing, but since the compiler lacks native support, the error messages you get most likely make it impossible to figure out what is going wrong. I think Grapefruit's Records, HList, Data.Accessor, etc.. might also work.</div>
<div><br></div><div>Any guidelines and comments regarding "strong duck typing"/"structural subtyping" are very welcome, since the lack of this is the only reason why I would prefer a dynamic language over a static one.</div>
<div><br></div><div>Thanks a lot,</div><div>Peter Verswyvelen</div><div><br></div></div></div></div></div></div>