4 ms·

This is so misinformed it isn't even wrong. * IO doesn't have to be wrapped in a monad, there are other models * Clojure cannot handle IO/side effects very we

by tel 12y ago

This is so misinformed it isn't even wrong. * IO doesn't have to be wrapped in a monad, there are other models * Clojure cannot handle IO/side effects very well (compared to any language with effect typing) * Input and output are not pure, but the `IO` type itself is pure. Rather, constructions of the IO type are pure and then the runtime can interpret IO values impurely. * The entire point is to separate the impure from the pure. It's not that you're forced to, it's that you desire to. * Monads are not, in themselves, generic ways to reason about state. They are far more general. * Anything that is not 100% pure is 100% impure. Without purity guarantees you cannot trust code you call upon to not do side effects. This breaks local reasoning.

taeric 12y ago
I can't help but disagree with your final bullet. You won't have "contractual and checked by the compiler" trust in code you call upon, but it is quite common to trust the code that you call in any language to do just what it claims it will do. Consider, do you really think that code was lacking local reasoning before the likes of Haskel? It is arguable that things were more difficult then, but the argument is still out that things are easier now.
- tel 12y ago
  Local reasoning is absolutely impossible unless you have some kind of contract which ensures that your (local) code is pure. This is almost definitional. What I haven't claimed is that no other fragment of code in another language can be pure. In nearly any language (1+1) is pure. My point was that literally any impurity inside of a fragment of code makes the whole thing impure (in most cases) and therefore destroys local reasoning. The "in most cases" bit above is important because there are ways to "purify" a code fragment so that code which uses it cannot witness the impurity inside and therefore it restores local reasoning "above" that level. The ST monad is such an example.
  - taeric 12y ago
    I don't think "absolutely impossible" means quite what you think it means. Again, I will make no claims that it is easy. And in some cases you may wind up with some global reasoning entering into the coding process. Honestly, with how many solutions I've seen with tons of "locally pure" parts that were a bloody mess to deal with, maybe some "global" reasoning is called for.
    - tel 12y ago
      I'm willing to consider that I might be wrong, but here's the argument. If I am looking at some code which includes computation (by which I include function calling but also reference access which is sometimes trickily ignored as a computation) then I cannot assess the behavior of this code without knowing either (a) the computation is side-effect free and therefore has a mere value semantics or (b) it is not and can potentially be affected by or affect non-local parts of the code. To hit case (a) I don't need a language which enforces purity, but I do need to know that everything "beneath" where I'm standing is pure. In this case, uncertainty, even tiny amounts of it, whittles away (a) entirely and leaves me in concern (b). I'm not saying that global reasoning is bad or infeasible, but I am saying that lacking purity you cannot trust local reasoning until you isolate the pure fragment. For instance, you might state that (!x + !y) involves the global reasoning of what the values of (x) and (y) are but is local reasoning otherwise. I'd argue that actually local reasoning is destroyed until you refactor this code as let x_value = !x in let y_value = !y in (x_value + y_value) where the parenthetical fragment is now pure and local as the side effects were sidelined into the let clauses. I previously wasn't saying that "locally pure" code is preferable to globally reasoned code. I'm not completely certain that I would say that in all cases. I feel very confident though that it's (a) the right default and (b) something that should be used to a far greater degree than most code I see written which more or less demands global reasoning to do anything non-trivial at all.
      - taeric 12y ago
        First, I want to say that your last paragraph is something I do agree with. Sounds like we are ultimately on the same page and do actually agree with each other. My point was simply that local reasoning is strengthened by trust in everything that you do locally. This is actually no different than living. I trust that what I hand off for recycling is actually getting recycled correctly. I have no real verification of this, however. Now, you can work in a language that demands this for you. However, there are times where this demand actually makes things more difficult than they need to be. Conversely, there are plenty of times where not honoring this idea leads to annoyance. Again, I do agree with your final point. I'm just not clear on where empirical results lie on this. Too much of it is just a very compelling argument.

by tel 12y ago

taeric 12y ago

I can't help but disagree with your final bullet. You won't have "contractual and checked by the compiler" trust in code you call upon, but it is quite common to trust the code that you call in any language to do just what it claims it will do. Consider, do you really think that code was lacking local reasoning before the likes of Haskel? It is arguable that things were more difficult then, but the argument is still out that things are easier now.

tel 12y ago
Local reasoning is absolutely impossible unless you have some kind of contract which ensures that your (local) code is pure. This is almost definitional. What I haven't claimed is that no other fragment of code in another language can be pure. In nearly any language (1+1) is pure. My point was that literally any impurity inside of a fragment of code makes the whole thing impure (in most cases) and therefore destroys local reasoning. The "in most cases" bit above is important because there are ways to "purify" a code fragment so that code which uses it cannot witness the impurity inside and therefore it restores local reasoning "above" that level. The ST monad is such an example.
- taeric 12y ago
  I don't think "absolutely impossible" means quite what you think it means. Again, I will make no claims that it is easy. And in some cases you may wind up with some global reasoning entering into the coding process. Honestly, with how many solutions I've seen with tons of "locally pure" parts that were a bloody mess to deal with, maybe some "global" reasoning is called for.
  - tel 12y ago
    I'm willing to consider that I might be wrong, but here's the argument. If I am looking at some code which includes computation (by which I include function calling but also reference access which is sometimes trickily ignored as a computation) then I cannot assess the behavior of this code without knowing either (a) the computation is side-effect free and therefore has a mere value semantics or (b) it is not and can potentially be affected by or affect non-local parts of the code. To hit case (a) I don't need a language which enforces purity, but I do need to know that everything "beneath" where I'm standing is pure. In this case, uncertainty, even tiny amounts of it, whittles away (a) entirely and leaves me in concern (b). I'm not saying that global reasoning is bad or infeasible, but I am saying that lacking purity you cannot trust local reasoning until you isolate the pure fragment. For instance, you might state that (!x + !y) involves the global reasoning of what the values of (x) and (y) are but is local reasoning otherwise. I'd argue that actually local reasoning is destroyed until you refactor this code as let x_value = !x in let y_value = !y in (x_value + y_value) where the parenthetical fragment is now pure and local as the side effects were sidelined into the let clauses. I previously wasn't saying that "locally pure" code is preferable to globally reasoned code. I'm not completely certain that I would say that in all cases. I feel very confident though that it's (a) the right default and (b) something that should be used to a far greater degree than most code I see written which more or less demands global reasoning to do anything non-trivial at all.
    - taeric 12y ago
      First, I want to say that your last paragraph is something I do agree with. Sounds like we are ultimately on the same page and do actually agree with each other. My point was simply that local reasoning is strengthened by trust in everything that you do locally. This is actually no different than living. I trust that what I hand off for recycling is actually getting recycled correctly. I have no real verification of this, however. Now, you can work in a language that demands this for you. However, there are times where this demand actually makes things more difficult than they need to be. Conversely, there are plenty of times where not honoring this idea leads to annoyance. Again, I do agree with your final point. I'm just not clear on where empirical results lie on this. Too much of it is just a very compelling argument.