Comment by aidenn0

6 years ago

What does your dynamic language do when you take the first element of an empty list? There is no obvious "correct" thing to do. Furthermore, whatever you do return is unlikely to be the same sort of thing that is returned for a non-empty list.

A dynamic language will have a behavior that corresponds to some sort of type signature, and it's not possible to write behavior that corresponds with the type signatures given as examples of "impossible" in the article.

A type signature is merely a statement about behavior, so it's nonsensical to make a false statement about the behavior, and Haskell catches this.

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aidenn0

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noobiemcfoob  6 years ago

It throws an error. Errors are a type of behavior. Some of those error behaviors you can cope with. You catch those. Others you can't. You raise those and either the system can cope or it can't and you crash. What part is nonsensical?

  • aidenn0  6 years ago

    Throwing an error should be part of the function signature. Otherwise the statement "This function takes a list of objects of type A, and returns an object of type A" is false; sometimes it will return an object of type A, other times it will signal an error.

    • noobiemcfoob  6 years ago

      It's better to assume all code could throw an error. No statement is safe. This becomes particularly important in distributed computing as resources may be offline at any given moment. It's from this perspective that type systems afford a false sense of security and appear to be scratching the itches of overactive Type As.

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millstone  6 years ago

What would be the type signature of, say, Python's `pickle.load()`?

  • aidenn0  6 years ago

    In the style TFA you should wrap pickle.load() with a function that will unpickle the specific type you are expecting. So you should write a unpickleArrayOfInts() or whatever.

    The actual type would be a rather large union type, which would be unwieldy to use (but in an untyped system like python you still would need to deal with all of those corner cases to have a program that is correct in the face of arbitrary input).

    Really the biggest annoyance of type systems is that they make you deal with corner cases that you don't think are practically possible. If you are right, then they are wasting your time. If you are wrong, then they are saving you from having bugs.