On Sat, Jul 18, 2020 at 1:55 PM Markus Heukelom <markus.heuke...@gain.pro> wrote:
> Concerning the current generics proposal, was it every considered to not > allow type contracts at all? > > No type contracts would mean that generic functions can only use =, & on > variables of parametric types, as these are always available for all types. > Nothings else is allowed. > > I've written a lot of code in this style in Standard ML. In fact, SML distinguishes between types and eqtypes, where the latter allows for equality comparisons. Many types do not have an equality check defined on them, most notably floating point values and functions among the ground types. The place where you hit the limitation is usually when you have an ordered tree and you need a function `cmp` of type `'k -> 'k -> order` which can order the keys in the tree. When you construct the tree, you have to provide such an ordering function and embed the order in the data structure of the tree. It is usually nicer to have that order implicitly bound to the type, especially for trees where you don't require a specific order, only some order that is total (or perhaps well-ordered). Standard ML gets around this limitation by the means of "Functors" which are functions from modules to modules. In Go-parlance that would be generics on the package level, which I believe has been explored. My guess is that to make this efficient in writing, you also need to have nested packages, something which Standard ML has, but Go doesn't. Functors are also a nominal construction, whereas Go tends to use structural constructions where possible. The current Go solution is closer to Haskell, where you can write the signature for the insertion into a tree as insert :: Ord k => k -> a -> Map k a -> Map k a that is, given a key, k, a value of type a, and a map for those types, produce a map where the K/V pair has been inserted. The `Ord k => ...` part constrain the type `k` to be ordered. -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/CAGrdgiX8EnOdq38cLwg3jsqK7-k7Vv9GKq1qXChk1VATajDfpw%40mail.gmail.com.
