Re: [Haskell-cafe] Re: [Haskell] View patterns in GHC: Request?for?feedback

On Fri, Jul 27, 2007 at 09:02:42AM -0500, Jonathan Cast wrote:

On Friday 27 July 2007, Jon Fairbairn wrote:

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ChrisK writes:

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Because this is starting to sound like one of the maddening things about C++.

Namely, the automatic implicit casting conversions of classes via their single argument constructors.

Unfortunately I'm not sufficiently familiar with C++ to know what this means. Perhaps you could clarify?

Somebody noticed that, in C, you could mix integers and floats (almost) freely, and in Classic C, you could mix pointers and integers freely, and thought this was /such/ a wonderful idea that C++ has special syntax to declare the conversion functions allowing you to, say, mix pointers and pointer-like classes freely, or to mix char*s and strings freely, etc. It's what makes

To give a somewhat more mundane example if you define a class Array class Array { public: Array(int); // ... construct a new array of specified length ... } Then if you make the mistake of passing an integer constant to a function that expects an Array, C++ will happily construct a new Array of that size and pass that to the function. Even more exciting when you use overloading: if you define multiplication between two Arrays, then if you accidentally try to multiply an Array by an integer constant (thinking it'll be a scalar multiply), then a new Array of that size will be constructed and multiplied--almost certainly resulting in a runtime error (mismatched Array sizes), but certainly not what you want. The solution is to add explicit to the constructor for all single-argument constructors (except perhaps occasionally when you actually want explicit construction of objects). The reasoning behind this, of course, is to allow nice interactions of home-made classes such as complex numbers, or string classes (which you might want to be automatically constructed from string constants). -- David Roundy Department of Physics Oregon State University