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sml

convertion between record and tuple?

Now that tuple is just record in SML, I tried:

enter image description here

The codes above I hope this record can accept a tuple, but it reports an error.

So, there's no implicit conversion from tuple to record and from record to tuple(though didn't post here, I have tried at local actually).

My question is why SML doesn't supply such conversion?(because these conversions are quite frequent in c++)

Solution

  • Tuples are records with members named 1 through n. That is, the three element tuple (a, b, c) is really a record {1 = a, 2 = b, 3 = c}. So there doesn't need to be an implicit conversion between tuples and records because they're already the same thing.

    Now the function you defined takes a record with the members x, y and z and you pass it a record with the members 1, 2 and 3. This doesn't work. For this to work there'd need to be an implicit conversion between records with different member names, which seems like a very bad idea.

    (because these conversions are quite frequent in c++)

    There are no implicit conversions in C++ between unrelated structs or classes with different members (or even the same members). To do that, you'd need a reinterpret_cast (or a C-style pointer cast on the address), which is neither common nor a good idea (and wouldn't work with anything but POD types).