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Meeting weekly 2013 01 15

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dherman edited this page Jan 15, 2013 · 2 revisions

Attending

dherman, azita, nmatsakis, jclements, pcwalton, brson, tjc

Agenda

  • FUEGO!
  • topics on Patrick's whiteboard:
    • move assert and log to macros
    • conditions in #[cfg()]
    • fn() unsafe { ... }
    • where should we allow trait bounds?
    • unsafe pointer indexing
    • [ u8 * CONST ]
    • removal of "both type and trait" impls
    • Eq and Ord reform

Tree's on fire

  • Graydon: tree's been on fire for over a week
  • Patrick: none of us are hitting these LLVM issues and I don't understand them

assert and log

  • Patrick: still built-in; doesn't seem necessary
  • Graydon: agreed
  • Patrick: question with assert! makes it look like assert-not; had an idea whereby you could rename it failUnless but don't know whether people like that
  • Patrick: bang doesn't look like not
  • Graydon: wouldn't worry; think you have to understand id! means macro
  • Dave: I agree with graydon
  • Patrick: ok (consensus)
  • Graydon: good luck figuring out how to do log
  • Patrick: why hard?
  • Graydon: flags
  • Patrick: syntax extension, probably not macro
  • Graydon: introducing syntactic elements that don't exist in language; we don't have mutable globals
  • Patrick: interesting. we do have const in foreign mods; that lets you have unsafe reference to external; all we need is some way to define unsafe definition of an external
  • Brian: const gives you mutable reference?!
  • Patrick: immutable one
  • John: what is the feature involved here?
  • Graydon: global variables
  • John: I'll just read up
  • Graydon: logging is currently controlled by global variables injected by compiler
  • Patrick: we can have the global variable in the runtime, write it in .c pending figuring out how to do it in Rust
  • Graydon: but it's global per-module
  • Graydon: this is why we've been blocked on this forever
  • Patrick: I think we're gonna need to have unsafe globals for interacting with C code anyway
  • Graydon: mostly agree
  • Patrick: we'll figure it out

conditions in #[cfg()]

  • Patrick: only have or right now; would be nice to have conjunction and negation
  • Patrick: I propose: if you specify multiple conditions they're anded, and you can say not(--)
  • Patrick: that's minimal. we could have and and or operators
  • Graydon: comma-separated seems fine
  • Patrick: basically gives you disjunctive normal form
  • Brian: agree

function unsafe

  • Patrick: diff between fn unsafe and unsafe fn -- originally we were like what?
  • Patrick: general chatter was this wasn't great
  • Brian: I tried to remove it once, didn't like it
  • Brian: wouldn't mind removing it though
  • Patrick: I never write fn unsafe
  • Graydon: I'd be in favor of removing
  • Patrick: just a form that only exists in statement or expression position
  • Brian: not gonna lose any sleep if you remove it
  • Patrick: sympathize with brevity, but I worry this is the one form that needs to be unambiguous (consensus)

trait bounds

  • Patrick: currently allow in weird places like structs
  • Patrick: don't enforce them there
  • Niko: we do, probably not correctly, but we try to
  • Patrick: feel like having a trait bound in data (structs, enums, etc) is kind of weird overlap of functionality of trait bounds in code (fn, impl)
  • Brian: can't use them till you call a function
  • Patrick: don't really serve any purpose
  • Niko: can't ever construct an instance that's not e.g. Eq-able
  • Niko: when we had class syntax they made more sense
  • Brian: user could create your data type, violating some of your intended invariants
  • Patrick: if you export the constructor
  • Patrick: if not, you control it
  • Niko: if you export the type it has a literal form
  • Niko: we had this discussion once before I thought
  • Graydon: feel like we've discussed this
  • Niko: if we ever want them we can put them back
  • Graydon: no one's gonna put anything there in the syntax anyway, totally future-proof

unsafe pointer indexing

  • Patrick: ability to index into unsafe pointers -- people want this
  • Brian: never felt need for it; offset method gives you offset
  • Niko: no strong opinion
  • Patrick: maybe let's not worry about it
  • Graydon: why can't use index method?
  • Patrick: unsafe; index method doesn't have unsafe bound on it
  • Niko: you have to build it in if you want it
  • Niko: can certainly support + and arithmetic
  • Niko: I don't really care
  • Graydon: discourages people from doing it, which is good
  • Patrick: okay, let's not do it

u8 * const

  • Patrick: every other day someone comes in and asks how to create a fixed-length vector with a const
  • Patrick: concerned about cycles: consts are type-annotated, so creates weird cycle between type resolution and const evaluation
  • Patrick: given thought in different ways. way I like best: remove field projection from const evaluation language, then nice bottom-up phenomenon: means you can evaluate all the numeric constants without evaluating any other constants
  • Niko: do we have array-indexing?
  • Patrick: have to remove that too if so
  • Dave: still have to do cycle detection between const variable declarations, right?
  • Patrick: yes, already do. const-and-type cycles is the hard one
  • Niko: I'm in favor of this
  • Graydon: I'm in favor of this, if you talk about removing those two operators from the integer constants
  • Graydon: if you look at work in constants, there's three expression grammars: full, const, and int const expressions -- latter are ones that can serve as enumerator values; they're the ones that should be here as well; array sizes should only be int const expressions
  • Niko: not just property of expression that appears in that position, but any variables referenced by that expression, right?
  • Graydon: yes, it's a deep property
  • Patrick: this exists today?
  • Graydon: believe if you look at constant classification code
  • Niko: guess that's a compromise: evaluate those constants that appear in those positions early... (cut off)
  • Patrick: all consts that have scalar values, just make them always integer?
  • Niko: didn't think that's what he's saying. think he's saying const of some types can use field projection, other types not
  • Niko: seems weird
  • Dave: not clear what Graydon means, let's let him explain G:
  • Niko: Four examples:
    // Example 1:
    const foo: uint = ...;
    let x: [int * foo] = ...;'

    // Example 2:
    struct S { f: uint }
    const foo: S = ...;
    let x: [int * foo.f] = ...;'

    // Example 3:
    struct S { f: uint }
    const foo: S = ...;
    const bar: uint = foo.f;
    let x: [int * bar] = ...;'

    // Example 4:
    struct S { f: uint }
    const foo: S = ...;
    const bar: uint = foo.f;
    /* bar is never used in a type context */

    // Example 5:
    struct S { f: uint }
    const foo: S = ...;
    const bar: S = S { f: foo.f };
    
    // Example 6:
    struct S { f: uint, g: [u8 * bar] }
    const foo: S = S { f: bar, g: [ 1, 2, 3, 4, 5 ] };
    const bar: uint = foo.g[0];
  • Graydon: all I'm saying is int constant category is what we should use in array bounds
  • Niko: seems to me that's insufficient. have these 4 examples; what I'm concerned about is... sounds like bad case is ex 3: expression is simple, but definition of bar requires a field projection
  • Niko: difference between that and ex 4 is constant is never used in a type context; patrick said maybe all uints should be simple, but that's pretty restrictive
  • Dave: concerned the distinction between simple/non-simple expressions will be hard to understand, and the errors will be hard to diagnose
  • Graydon: why does turning off subsections of grammar make the problem any easier? you still have to check for cycles, right?
  • Patrick: but then you have to check for cycles between constants and types
  • Graydon: but there's no way for a constant to be defined in terms of a type
  • Patrick: but constants have to be type-annotated; type checker currently works by doing type collection first -- that now has to be intertwined with constant-evaluation
  • Dave: oh, cycle example: const i: [ u8 * i ] = ...;
  • Niko: we can detect these cyclic conditions, but I think what patrick's objecting to is big phase that does types and constants altogether
  • Niko: in patrick's scheme they wouldn't be intertwined
  • Graydon: well, int constants
  • Niko: but int constants can refer to arbitrary types
  • Niko: if you remove field projection, int constants only ever refer to int constants
  • Graydon: I'm fine with that restriction in int constants, which are the only thing involved in this early pass
  • Niko: seems weird to me to have different legal expressions based on type of constant
  • Graydon: no, it's already true. because & is legal here. int constants get put in memory in a place
  • Niko: don't understand
  • Graydon: every constant that is not an integer constant is semantically and importantly semantically different. not just a value, but a place in memory. has an address that can be taken
  • Niko: that's not necessary
  • Graydon: it is necessary
  • Niko: why? could take address of any constant
  • Graydon: couldn't use in an integer constant in a type
  • Graydon: can cast pointers to integers
  • Niko: don't think we allow that in safe code, do we? we shouldn't
  • Niko: don't think you can cast &T
  • Graydon: uncomfortable pretending addresses don't exist
  • Niko: can find out what they are through unsafe code. it's unsafe if we go to moving GC, for example
  • Niko: so you'd be happier if constants with integral type are special category
  • Dave: if there are important semantic distinctions, there should be important syntactic distinctions to make that clearer
  • Patrick: not totally convinced of utility of field projection in constants anyway
  • Niko: not convinced about field projections. seems like you could pull out that value...
  • John: error message is gonna be unfriendly: b/c this thing turned out to be int, not allowed to do field projections
  • Graydon: this is completely normal in C-family languages
  • Graydon: they have sizeof
  • Patrick: that's not a field projection
  • Graydon: same effect, creates backwards projection with cycles
  • John: doesn't that argue for Dave's scheme?
  • Niko: argument we should just do the combined phase.
  • Dave: if we do that, don't need the syntactic distinction as much
  • Patrick: maybe we should just combine
  • Niko: not so sure. would also require that we type check constant expressions, b/c we have to be able to evaluate them.
  • Niko: right now: first we create the types, then can type check
  • Dave: could sort of do "dynamic typing" -- check as you const-evaluate
  • Niko: seems painful, anyway
  • Patrick: could create graph of types and topologically sort, that gives you an evaluation order
  • Dave: this is likely to get over-complicated
  • Graydon: lemme play devil's advocate against my own position. we've been conservative b/c it's hard to decide. one reason we've resisted sizeof so long -- it's junky. target-dependent. and it's not even enough; you end up with alignof... we have difficulty with native C libraries that present partial types or whose size depends on size of other types; don't think those patterns are gonna go away.
  • Graydon: anyway I agree this complicates the type checker
  • Patrick: C doesn't have cyclic dependencies so it's easier for them
  • Niko: do we use field projection in const-expressions?
  • Patrick: problem is sizeof really
  • Niko: not so bad b/c you know what its type is
  • Patrick: no you don't
  • Niko: you can scan for sizeof in AST and find it; induces a dependency, yes, but I don't think it's... not same thing. doesn't require you have full...
  • Graydon: seems like a chat with community might help, background research on other languages
  • Graydon: don't have quite enough to decide right now
  • Patrick: ok, that's fine
  • Graydon: I'm not wedded to field projection, but it's a bigger problem
  • Patrick: that's completely right, sizeof is also important to consider

removal of both type and trait impls

  • Patrick: if you're in same module as T and you write impl T : tra then methods in that impl are available in both the type and the trait, but not if you're in a different module
  • Patrick: this behavior is surprising, will probably lead to name conflicts
  • Patrick: would like to remove it
  • Brian: agree
  • Graydon: agree
  • Niko: agree

Eq and Ord reform

  • Patrick: don't quite know what to do
  • Patrick: design space
  • Patrick: currently Eq and Ord are good for partial but not so good for total. deriving Ord is not particularly efficient since lessThan and eq are different methods.
  • Niko: I think the efficiency question is not even necessary. correctness of hashtables is based on...
  • Patrick: compare the efficiency of these two approaches:
    // C++-like Ord
    impl<T:Ord,U:Ord> (T,U) : Ord {
        fn lt(&self, other: &(T,U)) -> bool {
            if fst(self) < fst(other) {
                true
            } else if fst(self) > fst(other) {
                // two comparisons between fst(self) and fst(other) :(
                false
            } else {
                snd(self) < snd(other)
            }
        }
    }
    
    // Haskell-like Ord (also Java)
    impl<T:Ord,U:Ord> (T,U) : Ord {
        fn ord(&self, other: &(T,U)) -> Ordering {
            match fst(self).ord(fst(other)) {        // one comparison
                Less => Less,
                Greater => Greater,
                Equal => snd(self).ord(snd(other))
            }
        }
    }
  • Patrick: correctness of hashtables: if you put NaNs as key in hashtable, you're going to have pain
  • Dave: haha, dealing with this in JS too
  • Patrick: because NaN != NaN
  • Patrick: fundamental issue is that there's partial vs total, two different interfaces
  • Patrick: hash tables and binary search trees want total equality and total ordering, respectively
  • Patrick: for ordinary arithmetic operators, you typically want the partial equality and partial ordering
  • Brian: why do you want that?
  • Patrick: 1) expectation based on IEEE, 2) efficiency in hardware
  • Dave: the hardware point is the compelling one, trumps everything
  • Patrick: C++ says putting NaN in hashtable is undefined behavior; always use partial equality
  • Patrick: Haskell says: we are not IEEE 754, everything is total
  • Patrick: as a result, not sure they can use most efficient fp hardware
  • Patrick: neither seems particularly ideal. feel that both are suboptimal
  • Dave: sounds like you want 4 interfaces; worry about complexity
  • Patrick: other problem: what do the operators mean
  • Patrick: derive PartialEq and TotalEq, one for operator, one for hashtable; not great
  • Patrick: also can't use operator for the impl of hashtable, could be slow
  • Graydon: I'm not following. why does hashtable want total equality? why is it undefined in C++?
  • Patrick: if you put NaN in, can't get it out again
  • Graydon: what does a normal C++ library actually do?
  • Patrick: don't know
  • Niko: depends very much on precise way they wrote that loop (e.g., whether they use == or !=)
  • Graydon: when you say partial equality, you mean one where they can both be false?
  • Niko: yeah
  • John: wait, does NaN != NaN return true or false?
  • Dave: true
  • Dave: I tend to think that the nature of floating point is that it's hazardous
  • Patrick: not so simple because of example code above
  • Patrick: wait, for binary search trees, you really want the ord method, not less-than
  • Patrick: proposal: Three traits:
    • Eq is a partial ordering.
    • Cmp gives you the operators <, <=, >=, and >, and is partial.
    • Ord is the "ord" function and is a total ordering.
  • Brian: I don't understand why this is better if hashtables are still broken for floats
  • Patrick: here's why I like this: < is not what you want for binary search trees; they want ord. and ord can be efficiently implemented for substructures
  • Brian: so regardless of float problem want ord
  • Niko: arguably could use ord for hashtables
  • Niko: float would be instance of Ord using Java ordering
  • Brian: because Ord provides Eq, doesn't matter if Eq is total
  • Patrick: I'm not sure about solving for hashtables
  • Brian: thought that was whole motivation
  • Patrick: there are many. biggest is that I can't implement deriving Ord efficiently
  • Brian: okay. only matters for hashtables. everyone else uses Cmp. those are still gonna be same thing as always. where does this matter? who will use Ord?
  • Patrick: binary search trees
  • John: current proposal is hash requires Eq or Ord?
  • Patrick: prefer Eq and don't care about NaNs
  • Niko: me too
  • John: could be security issue. could get back somebody else's value if you ask for NaN
  • Niko: how?
  • John: no, I take it back
  • Dave: would urge us to have very clear, carefully written documentation clearly explaining when to use Cmp vs when to use Ord -- otherwise people will find it confusing
  • Dave: should Ord <: Cmp? since they're related
  • Patrick: no because ord doesn't use less-than
  • Niko: hm, unclear... they could
  • Patrick: this solution doesn't solve float at all
  • Patrick: sort of hackishly solves floats with Ord
  • Patrick: if we really wanted to solve it, we'd have TotalEq and PartialEq
  • Dave: haven't understood the deriving part of this issue
  • Patrick: want deriving to generate optimal code, but I can't without Ord
  • Dave: sounds like you should give this some more thought and present it next week
  • Niko: or write it up. I think this is the most plausible solution
  • Dave: no relationship between Cmp and Ord?
  • Niko: yes. reason is float -- have both a partial and total ordering, so can't just have one trait hierarchy
  • Dave: really think we should have a well-written rationale as well as clear guidelines for use
  • Niko: yeah
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