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Not to be too nit-picky, but it doesn't actually have a for-loop. The for you linked to is not a for-loop but a for-each loop (simply called for). Well it is ac

Not to be too nit-picky, but it doesn't actually have a for-loop. The for you linked to is not a for-loop but a for-each loop (simply called for). Well it is actually more restricting even then a for-each loop, as it maps over elements and can only return a collection of the same length as it iterates over. That said, you're correct it does have a pseudo while-loop. Though it will still force you to use a managed mutable container along with it, which is still safer than without. Also, it's a very ackward while-loop, as it doesn't support continue or break, that's because in reality it isn't a real while-loop, it's a fake while-loop built on top of a recursive loop. And for all other readers, it's true, Clojure doesn't disallow the use of less abstract less safe construct, it simply makes them more ackward to use and not the default and obvious choice. The reason for this is also something the article didn't bring up, but higher level safer abstractions often come with a performance cost. This is why most languages have escape hatches for whatever safety mechanism they provide, and why Rust lang loves to talk about "zero-cost" abstractions.

dkersten 6y ago
Clojure's "for" is really a list comprehension function, but Clojure's "doseq" is more or less what you'd expect from a for loop (like Python's anyway or any other range-based for).
- didibus 6y ago
  Python's for isn't a for-loop either, it's a for-each loop also sometimes called a for-in loop. In that sense, neither Python nor Clojure have real for-loops. But Python gets much closer, because Clojure's doseq is similar to its while, it is actually syntax sugar for a recursive loop, and not an imperative loop. While Python's for is an imperative for-each loop, which supports the statements break, continue and return inside it.
  - dkersten 6y ago
    doseq isn’t imperative because it doesn’t support break or continue? That doesn’t make sense.
    - didibus 6y ago
      Oh sorry, I meant to say iterative, not imperative. Basically all loops in Clojure are implemented in terms of recursion (loop/recur) and not jumps. Which is why there's no break, continue or return statements. Whatever categories you assign them in, I was more trying to show the differences between Python's for and Clojure's doseq. And also show that neither are your classic for-loop.
      - dkersten 6y ago
        It is iterative too, though: it iterates through an input sequence. > Whatever categories you assign them in, I was more trying to show the differences Fair enough. I take your overall point, I just don't think that not being a classic for-loop matters so much in practice, certainly not "most of the time" anyway.
        didibus 6y ago
        > It is iterative too, though: it iterates through an input sequence. I don't think these words have supper formal definitions to be honest. I'm using iterative to contrast it against recursive. So in that respect, Clojure's doseq is recursive. You can also see it as iterating over a sequence and thus say it is iterative, but the way it loops over the sequence is through recursion, which is where doseq differs with Python's for. Maybe it's clearer if I say that doseq is recursive while Python's for isn't and just not mention iterative at all haha. > Fair enough. I take your overall point, I just don't think that not being a classic for-loop matters so much in practice, certainly not "most of the time" anyway. I don't know what you mean by "matter", but with regards to the article I'm discussing I'd say it does. A classic for-loop is prone to easy to make bugs that are well known, such as "off by one errors", and "overflows". So the fact that the language has you use this higher level abstraction for looping over collections can help prevent a certain amount of such errors and thus provide additional program safety. Now I don't know if the further differences between Clojure doseq and Python for would also result in safer code. I guess the question is: is the use of break and continue a common source of defect? And is forcing branching instead a safer alternative? Personally I don't know, I'd say maybe not. That said, Clojure also favours the use of its immutable for or reduce over doseq, and that is arguably much safer, because mutable state inside a for-each loop is also a known source of common defects, like changing the sequence as you loop over it, and especially if concurrency is involved. So that's another relevant practical difference in my mind.

dkersten 6y ago

Clojure's "for" is really a list comprehension function, but Clojure's "doseq" is more or less what you'd expect from a for loop (like Python's anyway or any other range-based for).

didibus 6y ago
Python's for isn't a for-loop either, it's a for-each loop also sometimes called a for-in loop. In that sense, neither Python nor Clojure have real for-loops. But Python gets much closer, because Clojure's doseq is similar to its while, it is actually syntax sugar for a recursive loop, and not an imperative loop. While Python's for is an imperative for-each loop, which supports the statements break, continue and return inside it.
- dkersten 6y ago
  doseq isn’t imperative because it doesn’t support break or continue? That doesn’t make sense.
  - didibus 6y ago
    Oh sorry, I meant to say iterative, not imperative. Basically all loops in Clojure are implemented in terms of recursion (loop/recur) and not jumps. Which is why there's no break, continue or return statements. Whatever categories you assign them in, I was more trying to show the differences between Python's for and Clojure's doseq. And also show that neither are your classic for-loop.
    - dkersten 6y ago
      It is iterative too, though: it iterates through an input sequence. > Whatever categories you assign them in, I was more trying to show the differences Fair enough. I take your overall point, I just don't think that not being a classic for-loop matters so much in practice, certainly not "most of the time" anyway.
      - didibus 6y ago
        > It is iterative too, though: it iterates through an input sequence. I don't think these words have supper formal definitions to be honest. I'm using iterative to contrast it against recursive. So in that respect, Clojure's doseq is recursive. You can also see it as iterating over a sequence and thus say it is iterative, but the way it loops over the sequence is through recursion, which is where doseq differs with Python's for. Maybe it's clearer if I say that doseq is recursive while Python's for isn't and just not mention iterative at all haha. > Fair enough. I take your overall point, I just don't think that not being a classic for-loop matters so much in practice, certainly not "most of the time" anyway. I don't know what you mean by "matter", but with regards to the article I'm discussing I'd say it does. A classic for-loop is prone to easy to make bugs that are well known, such as "off by one errors", and "overflows". So the fact that the language has you use this higher level abstraction for looping over collections can help prevent a certain amount of such errors and thus provide additional program safety. Now I don't know if the further differences between Clojure doseq and Python for would also result in safer code. I guess the question is: is the use of break and continue a common source of defect? And is forcing branching instead a safer alternative? Personally I don't know, I'd say maybe not. That said, Clojure also favours the use of its immutable for or reduce over doseq, and that is arguably much safer, because mutable state inside a for-each loop is also a known source of common defects, like changing the sequence as you loop over it, and especially if concurrency is involved. So that's another relevant practical difference in my mind.