4 ms·

Aren't IO Actions just a list of things to do and changes to make? I'm asking genuinely to try to understand the difference.

• daxfohl 10y ago
  Yes they are, but I think "for loops" and "mutable lists" a'la Java's ArrayList and Vector and C#'s List<T> don't exist even in IO (I'm not a Haskell expert). That kind of thing has to be done with recursion. They exist in F# but are not idiomatic. However sometimes they're more intuitive (to me at least).
  • acomar 10y ago
    forM is a for each loop over anything iterable (Traversable in Haskell) that will execute a given monadic action on each element in the collection. A more traditional for loop is this over a range of integers or whatever else you feel like. forM (range 5) println Mutable collections exist in various forms depending on what kind of mutability you want. For example, there's transactional mutability that gives you mutable data structures inside an STM but forbids IO. And there's IO mutability that removes all the bounds but restricts you to the IO Monad. TVars, MVars, and the various kinds of Refs are what you're looking for here.
    • jbooth 10y ago
      If someone is asking for mutability for reasons of performance and clarity, giving them STM is like a kick in the teeth.
      • codygman 10y ago
        How is STM like a kick in the teeth? In performance? How? In clarity? How? Can you give examples of STM being a kick in the teeth versus mutability.
        • carterschonwald 10y ago
          I suspect he didn't keep the size of his stm atomically programs small. The docs and associated papers and book materials all make it very very clear that you want to keep stm transactions small and try to defer computation into thinks that are only evaluated when the transaction succeeds.
          • jbooth 10y ago
            Compare the cost of optimally, expertly built STM-using code to, say, just assigning some byte values in a *char array. Instruction count? Cache locality? Allocations?!? EDIT: Oh, and isn't STM typically just a wrapper over immutable stuff anyways? So he said "immutable copying is too slow for me", and you said "here, use this instead, it has all the copy overhead plus some additional overhead".. that's a kick in the teeth.
            • carterschonwald 10y ago
              ... Stm in ghc is definitely mutable. Before commit it has an oplog representation but it's most assuredly doing very imperative things like find grained locking and writes to the same reference :)
              • jbooth 10y ago
                Ah, well that's super cool. Doesn't completely counteract what I was saying but that's interesting, thanks for the tid bit.
            • 15155 10y ago
              > Compare the cost of optimally, expertly built STM-using code to, say, just assigning some byte values in a *char array This is comparing the wrong tasks. Apples and oranges. STM is usable for this particular task, but will almost assuredly be far slower. If performing the exact same task in Haskell, you could use IO and mutable memory for comparison. Where STM really shines, however, is in highly-concurrent environments. There's no great "C comparison" for hundreds of concurrent operations locklessly reading/writing to/from the same memory. STM makes this safe and performant without a change in semantics or additional mental overhead from the "non-concurrent" case.