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Comment by bhickey | Hacker News Reader

    Lazy languages like Haskell make it a point to be
    referentially transparent, i.e. making sure that the
    order in which expressions are evaluated will never
    affect their result.

Isn't referential transparency a consequence of purity, not laziness?

Edit: While I believe the quote is saying (albeit in a peculiar way) that lazy languages must be referentially transparent, I don't see how this particular response was arguing in favor of laziness. Referential transparency is great as an optimization enabler, which provides one compelling case for purity.

Meanwhile I'm working on a nascent project to develop an eager, pure language that targets LLVM.

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>I don't see how this particular response was arguing in favor of laziness.

In a referentially transparent language, you don't care about order of evaluation because whatever the order you will always end up with exactly the same result. Thus, losing control of it through laziness doesn't matter.

However, IMHO, the initial argument is kind of wrong. Laziness doesn't really make you lose control of order of evaluation. You know when expressions are going to be evaluated : when they are going to be needed. Obviously, when you use a lot of abstraction you didn't write like in Haskell, it becomes tricky. But, if you perfectly know how your data structures are processed by the compiler, you can safely use side effects with laziness. Not that you should, but you can.

RT needs to be seen in the context of reasoning about programs.

Suppose we want to define a constant function f such that f x = 0. Under strict semantics in a total language, you're fine. Under strict semantics in a partial language f undefined blows up. So constant functions, the simplest kind possible, don't even exist! OMGWTFBBQ!

You can still reason about programs, but at a very low-level, i.e. on operational semantics. RT is high-level reasoning. Math, which does have constant functions, seems pretty successful, so we'd like a slice of that cake, please.

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