March 25, 2008Const unbackled
I might have a function to test some property of a value. I expect to usually need this function inside some other functions. Since I know this function does not change its input data, I now want to _advertise_ this fact, according to the prevailing /mode du jour/. Let's say my function validates a string or a stream of UTF for internal correctness. In stand-alone usage it would look like vutf(s); // validate string or stream s, and throw if bad S sf = translateToFinnish(removeWhitespace(s)); But usually I'd use it inside something else, like S sf = translateToFinnish(removeWhitespace(vutf(s))); In the first case I discarded the output of vutf but want to be sure that s stays the same, in the second case I used it as input to the enclosing function. Since vutf does not modify its input, I should somehow be able to advertise this fact. I admit this example is contrived, but you get the idea. We should stop discussing whether/how this is implementable syntactically/semantically, and instead start discussing USE CASES, and DO WE NEED THIS IN THE FIRST PLACE. (One might also ponder on whether making deeply nested function calls in assignments is desirable in the first place.) UNLESS we think this out BEFORE ending up on a consensus on syntax/semantics, we might end up with what the C++ folks did with multiple inheritance. (They were too busy figuring out how to implement it, instead of whether there's any real-world use for it or whether it is implementable at all in a practical and usable way.) Use cases mentioned in the relevant NG discussions are checking for properties, returning slices, what else? Where is this indispensable?
March 25, 2008Re: Const unbackled
Posted in reply to Georg Wrede
"Georg Wrede" wrote > We should stop discussing whether/how this is implementable > syntactically/semantically, and instead start discussing USE CASES, and DO > WE NEED THIS IN THE FIRST PLACE. For starters, let's look at some functions in std.string: invariant(char) split(string s); invariant(char) split(string s, string delim); invariant(char) splitlines(string s); The use case could be said that one might want to pass a mutable string to these functions, and have it return mutable slices into the argument WITHOUT modifying the argument. string stripl(string s); string stripr(string s); string strip(string s); string chop(string s); These could all be usable on mutable strings, with the result being a slice of the original, and hopefully mutable. You should be able to specify that the function does not modify the argument. And in tango.text.Util: trim (source) // trim whitespace triml (source) // trim whitespace trimr (source) // trim whitespace strip (source, match) // trim elements stripl (source, match) // trim elements stripr (source, match) // trim elements chopl (source, match) // trim pattern match chopr (source, match) // trim pattern match delimit (src, set) // split on delims split (source, pattern) // split on pattern splitLines (source); // split on lines head (source, pattern, tail) // split to head & tail These all could be specified that source is not modified. -Steve