On Sun, Mar 8, 2009 at 12:45 PM, André Thieme <splendidl...@googlemail.com> wrote: > > I like that Clojure is a dynamically typed language. > Even in dynamic languages it is possible to find out a lot more > about the code itself than one may think on a first glance. > Clojure already supports type hints that can make code run > faster. But what if we could add a soft layer of static typing > into the compiler, by making promises to it, which can then be > enforced? > For example, we could promise that foo will always be a string: > (def+ foo "Hello Clojure") > > Same for functions, about its parameters and return values. > Note that I am not proposing a concrete syntax here, this is just > a sketch, so we can talk about it: > (defn reverse > "Returns a seq of the items in coll in reverse order. Not lazy." > [#^Collection coll] > (reduce conj () coll)) > > For all functions in the core.clj which don’t have side effects we > could make this promise. In that case the compiler could check > if foo is ever used in a position where not a string is expected. > And it could check if the return value of a function is an argument > to another function, and then see if those match: > > (+ (count foo) 15) ==> 28 > > (dec foo) ==> compile time error, can't subtract 1 from a String. > > (count (count foo)) ==> compile time error, as count returns a > number, not a collection. > > This would keep Clojure still extremly dynamic. One exception is: > functions and vars for which we gave promises can’t be changed > anymore during runtime. But these compiler checks would only > be made when in a form there only occur vars/functions which all > made promises about their types. These types can be inferred by > Clojure. For example in (def+ foo "Hi") we see that foo refers to a > string. As Clojure is mostly a functional language anyway it will > not cause much harm that foo can’t be changed anymore. > In principle each var that was def’ed should be seen as a constant. > As long we are debugging our code it can of course make sense > to change those vars. > > Anyway, with this proposal a good bit of code can be checked for > type correctness and there is another positive aspect: > the Clojure editors could make use of it. > When we write > (count<space> > the editor will not offer us auto-completion for vars that we promised > to be numbers or microwave objects. This feature would allow the > IDE to do more refactoring and more code completion at edit time. > > I am proposing a complete optional mode. Into the metadata one could > add a promise and state the explicit types, or add just an empty > promise > and have the Clojure compiler then to infer the correct times (it > could > give warnings if this were not possible). > This way only people who like these features could use them. > Even if the core.clj and the whole load of clojure-contrib changed in > a > way to fully support soft static typing, then still a user can decides > to > never put promises into his code, and so he wouldn’t even notice that > there is some (soft) mechanism for static typing. >
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