5 ms·

Sounds exotic at first glance, but how different is this from the Erlang implementation of tar parsing? I.e. how much of this is relying on Prolog-specific feat

• superdisk 4y ago
  The unique part is that you get encoding and decoding for free when just describing the spec. It is essentially just succinct head-clause matching though.
  • derefr 4y ago
    An idiomatic Erlang/Elixir implementation of validation for a binary format would also give you decoding for free — see e.g. https://zohaib.me/binary-pattern-matching-in-elixir/ https://zohaib.me/binary-pattern-matching-in-elixir/. So I guess it's the encoding part that's unique to how Prolog works. Which makes sense; my understanding is that in Prolog, insofar as a function encodingOf(A, B) is defined by using A and B on either side of a bind expression to equate B as the encoding of A, if encodingOf(A, knownEnc) would act as a parser for knownEnc, then encodingOf(knownDec, B) would act as an encoder for knownDec — essentially swapping the question "what would this parse to" given an encoded binary, for "what would parse to this" given a decoded term. Erlang can't do exactly that — a given variable's "bound-ness" is determined during compilation, and head-clause variables can't be indeterminately bound — but if it could, then you would indeed get an encoder for the same format "for free", since binary-pattern matching in Erlang becomes a binary-building expression depending on whether the variables in the expression are bound or unbound. (Now I'm curious how hard it would be to add indeterminately-bound compile-time function polymorphism to Erlang — i.e. generating bytecode for the powerset of bound-ness-es of the variables in the head clause, only where at least one generates valid bytecode, probably name-mangled with a bitfield suffix of said bound-ness-es; and then being able to bind function parameters from such functions as new output variables in expressions, where the call sites also generate a call to the equivalent mangled name. The usefulness of this would probably be pretty small, without the other aspects of Prolog, but you never know...)
    • Jtsummers 4y ago
      GP talks about getting encoding and decoding together by implementing the spec once. In Erlang/Elixir you can only get one at a time because you can't do the equivalent of this: some_predicate(<concrete values>, Decoded). % decoder some_predicate(Encoded, <concrete values>). % encoder some_predicate(<concrete values>, <concrete values>). % validator But in Prolog, that's what many predicates permit as a baseline. In Erlang, yes, a validator is, or can be, a parser/decoder. But it's not an encoder which still needs to be a separate function (or set of functions) to describe movement in the other direction.
      • dwohnitmok 4y ago
        This is fantastic bite-sized example of exactly what makes Prolog and other logic languages so different to program in and will probably become my go-to example for introducing what's so special about Prolog to programmers with backgrounds in other programming languages who are looking for examples that involve "real-world" programming tasks.
• lostmsu 4y ago
  To go even further, how much is it different from C# or C++? There you'd just declare the appropriate structs and bitcast.
  • superdisk 4y ago
    That works okay only for fixed-sized structs that don't require any transformation of their members-- otherwise you'll need to write a traditional parser.
    • lostmsu 4y ago
      It is there in Prolog version too. Search for "FileSize" on the page.
      • superdisk 4y ago
        What I mean is that if you need to parse a format that has variable-sized structures, you can't just memcpy them into a struct because structs have a fixed size. Additionally sometimes you need to parse a value into a more tractable representation, for example those octal ASCII strings in the tar format, which requires some additional processing that isn't possible with loading/serializing a struct.
        • lostmsu 4y ago
          Which is also there in the Prolog code.
  • Filligree 4y ago
    Sorry, isn't that undefined behaviour?
    • lostmsu 4y ago
      Not in C# for sure. Not a C++ expert, but I believe reinterpret_cast behavior is defined too.