9 ms·

>If your language has braces, then you are indenting for the sake of humans (because humans are good at noticing alignment and edges), and bracing for the sake

• vertex-four 12y ago
  > you have to cycle through all sensible indentations What? My editor indents stuff for me as per PEP8. I don't have to think about it. Why do you?
  • okasaki 12y ago
    def fun(): for x in xs: if a: b c There's no way for an editor to know where to re-indent c to. You have to cycle through the indents manually.
    • seanmcdirmid 12y ago
      Auto indent, tab, and shift tab are your friends. Really, it isn't that bad, considering the curly brace alternative is even worse from an effort perspective.
      • okasaki 12y ago
        To each their own. I came to C++ from Haskell and found braces to be superior (except visually).
    • vertex-four 12y ago
      How did you end up with c there, honestly? It's not a thing that's ever happened to me.
    • nothrabannosir 12y ago
      Perhaps I misunderstand, but vim, not a particularly uncommon editor, treats backspace at the start of a line by removing one indent level. So, you're at `b`, you hit enter: next line, same indent level. Now you hit backspace: indent level matches `if`. Again: indent level matches `for`. Type c, there it is. If you had curly braces, you'd use } instead of backspace. Literally the same number of keystrokes, no? Or were you talking about something else and did I misunderstand?
• seanmcdirmid 12y ago
  The computer is really quite proficient at inferring block structure from indentation, and if you use a modern editor this structure is manipulated rather effortlessly. The main problem with indent oriented languages is that not everyone is or wants to use a modern editor. Then there are LISPers who think braces everywhere are a swell idea.
  • marcosdumay 12y ago
    What's effortless is putting begin and end of blocks symbols only where they matter. Haskell is particularly guilty of creating several different indenting possibilities, and does require constant intervention. Python is less bad, but still requires more effort than C for example.
    • seanmcdirmid 12y ago
      You really have to design your language around it for it to work well. Haskell has a a lot of syntactic craziness in the name of simplicity.
      • masklinn 12y ago
        What? Haskell has a base language expressed in terms of braces and semicolons, and a relatively simple set of layout rules defining how the compiler can insert these braces and semicolons automatically.
        • seanmcdirmid 12y ago
          Right. I meant that Haskell has a whole lot of other weird syntax to consider, that make it hard to read compared to Python. It's use of indentation is not one of the problems.
          • codygman 12y ago
            Charitably speaking, that is very confusing considering you responded to: > Haskell is particularly guilty of creating several different indenting possibilities, and does require constant intervention with: > You really have to design your language around it for it to work well. Haskell has a a lot of syntactic craziness in the name of simplicity. Where "it" would seemingly be indentation, implying Haskell wasn't designed around making indentation simple.
• gnuvince 12y ago
  I like writing compilers, how do languages like Haskell and Python handle blocks-by-indentation? I figure you have some sort of INDENT token, but how do you know its width? Also, in specifying the grammar, with explicitly delimited blocks you just say `block := OPEN stmt_list CLOSE`. What about with indentation?
  • TazeTSchnitzel 12y ago
    I think they specify "INDENT foo DEDENT" and the lexer tracks indentation and issues INDENT/DEDENT as appropriate.
    • falcolas 12y ago
      This is correct. A bit of state (an indentation width stack) in the lexer creates the block boundaries as clearly for the interpreter as normal braces does. Any problems in parsing are typically only encountered by stateless parsers.
  • dochtman 12y ago
    I have one (probably quite naive) implementation here: https://github.com/djc/runa/blob/master/runac/parser.py#L54 https://github.com/djc/runa/blob/master/runac/parser.py#L54 (Runa uses tabs for indentation, not spaces, but it's basically the same thing.) Whitespace at the start of a line is a separate token, then I add a thin layer of processing that turns a change of indentation level (i.e. count chars in the token contents) into INDENT or DEDENT tokens.
• zak_mc_kracken 12y ago
  You're using a double standard, here: if the computer is good at indenting brace based languages, then it is equally good at indenting languages without braces just based on the grammar.
  • mindslight 12y ago
    Not at all. In this case, by 'languages without braces' we mean languages where indentation is part of the grammar. Python's grammar includes explicit INDENT/DEDENT tokens. It is impossible to reindent this mangled python: if z: print 'a' print 'b'
    • adrusi 12y ago
      Why would it be improperly indented in the first place? How is removing the indentation any different from removing braces? Sure, when you paste code somewhere that expects prose (Facebook status updates, for example), it will consolidate consecutive whitespace and thereby remove the indentation, but you shouldn't be pasting code into a place like that anyway, because no matter what you do it'll be unreadable, and most people aren't going to be interested enough to put it into their editor to autoformat.
      • mindslight 12y ago
        I did not make the value judgment you are arguing against.
    • mixmastamyk 12y ago
      Editors know that a python line ending in a colon must be indented and so indent it automatically. It wouldn't be hard to use the same method to reindent it.
      • unwind 12y ago
        No, lines following a line ending in a colon must be indented. But the OP's point is that you can't know how many lines make up the indented block, since there is no explicit block-end-marker left if the indent is gone.