<div dir="ltr">Tillmann,<div><br></div><div>Thanks, that is in interesting use case for handwritten Generics.</div><div><br></div><div>I'm not fully dissuaded though, simply because:</div><div><br></div><div>(1) it can't be too common! Especially when you intersect the people who have done or will do this with the people who care about SafeHaskell. (Again, if they don't, they won't mind this tiny Unsafe toggle.)</div>
<div><br></div><div>(2) even people who fall in this intersection still have the recourse of doing what they need to do and asserting "TrustWorthy". SafeHaskell is good at supporting this kind of individual exception.</div>
<div><br></div><div>Whereas in my case I have no recourse! Because my problem not about asserting that a particular module is TrustWorthy, but rather about keeping other users (running in -XSafe) from breaking my library.</div>
<div><br></div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Sun, Oct 6, 2013 at 6:54 PM, Tillmann Rendel <span dir="ltr"><<a href="mailto:rendel@informatik.uni-marburg.de" target="_blank">rendel@informatik.uni-marburg.de</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi,<div class="im"><br>
<br>
Ryan Newton wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
It is very hard for me to<br>
see why people should be able to make their own Generic instances (that<br>
might lie about the structure of the type), in Safe-Haskell.<br>
</blockquote>
<br></div>
I guess that lying Generics instances might arise because of software evolution. Let's say we start with an abstract data type of binary trees.<br>
<br>
module Tree (Tree, node, empty, null, split) where<br>
data Tree a = Node (Tree a) (Tree a) | Empty<br>
deriving Generics<br>
<br>
node = Node<br>
<br>
empty = Empty<br>
<br>
null Empty = True<br>
null _ = False<br>
<br>
split (Node a b) = (a, b)<br>
<br>
size Empty = 0<br>
size (Node x y) = size x + size y<br>
<br>
Note that this data type is not really abstract, because we export the Generics instance, so clients of this module can learn about the implementation of Tree. This is not a big deal, because the chosen implementation happens to correspond to the abstract structure of binary trees anyway. So I would expect that generic code will work fine. For example, you could use generic read and show functions to serialize trees, and get a reasonable data format.<br>
<br>
Now, we want to evolve our module by caching the size of trees. We do something like this:<br>
<br>
module Tree (Tree, node, empty, null, split) where<br>
data Tree a = Tree !Int (RealTree a)<br>
<br>
data RealTree a = Node (Tree a) (Tree a) | Empty<br>
<br>
tree (Node a b) = Tree (size a + size b) t<br>
tree Empty = Tree 0 Empty<br>
<br>
node x y = tree (Node x y)<br>
<br>
empty = tree Empty<br>
<br>
null (Tree _ Empty) = True<br>
null _ = False<br>
<br>
split (Tree _ (Node a b)) = (a, b)<br>
<br>
size (Tree n _) = n<br>
<br>
Except for the Generics instance, we provide the exact same interface and behavior to our clients, we just traded some space for performance. But what Generics instance should we provide? If we just add "deriving Generics" to the two datatypes, we leak the change of representation to our clients. For example, a client that serialized a tree with a generic show function based on the old Tree cannot hope to deserialize it back with a generic read function based on the new Tree. The size information would be missing, and the structure would be different.<br>
<br>
If we write a Generics instance by hand, however, I guess we can make it present the exact same structure as the derived Generics instance for the old Tree. With this lying instance, the generic read function will happily deserialize the old data. The size will be computed on the fly, because our hand-written Generics instance will introduce calls to our smart constructors.<div class="HOEnZb">
<div class="h5"><br>
<br>
Tillmann<br>
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</div></div></blockquote></div><br></div>