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By this logic you can write really fast programs in Haskell, because it avoids computing unused values (and therefore complete calltrees). (Unfortunately, the m

by pestaa 13y ago

By this logic you can write really fast programs in Haskell, because it avoids computing unused values (and therefore complete calltrees). (Unfortunately, the management of such calltrees -- thunks -- often has higher cost than outright computing them in the first place.) I try hard, but I'm not smart enough to write programs that do nothing. :)

mseebach 13y ago
You can do that in any programming language by, well, not computing unused values.
- chii 13y ago
  That's like saying anyone can make money by just making money. It's a null statement!
  - alcari 13y ago
    The term you're looking for is tautology, not 'null statement'.
  - mseebach 13y ago
    In the context of the comment I replied to, no, it's not. Haskell gives you some constructs that makes this easier but there exists no program whose Haskell implementations will run faster because of this, only programs that are easier to make fast.
    - nostrademons 13y ago
      The big advantage lazy evaluation has is composability. In an eager language, you have to explicitly code operations in a lazy fashion, which means that if you use a library, it will probably be eager. With Haskell you can be pretty sure that the runtime will evaluate only as much of the program as it needs to. That said, the downside is that it then becomes very difficult to reason about the performance characteristics of your library, because its performance depends on how it's used. Simon Peyton-Jones is on record as saying that laziness is probably the wrong default for a language - the big benefit for Haskell was that it "kept them honest" wrt purity, but in a production system you probably want strictness.
evincarofautumn 13y ago
You can. A lazy algorithm will do no more reductions than an eager one, and it will usually do fewer. You just have to specify strictness. The compiler is pretty conservative about adding strictness because it has to preserve behaviour. But if you know something will be computed, e.g., a loop accumulator, then you can just tell the compiler so.

by pestaa 13y ago

mseebach 13y ago

You can do that in any programming language by, well, not computing unused values.

chii 13y ago
That's like saying anyone can make money by just making money. It's a null statement!
- alcari 13y ago
  The term you're looking for is tautology, not 'null statement'.
- mseebach 13y ago
  In the context of the comment I replied to, no, it's not. Haskell gives you some constructs that makes this easier but there exists no program whose Haskell implementations will run faster because of this, only programs that are easier to make fast.
  - nostrademons 13y ago
    The big advantage lazy evaluation has is composability. In an eager language, you have to explicitly code operations in a lazy fashion, which means that if you use a library, it will probably be eager. With Haskell you can be pretty sure that the runtime will evaluate only as much of the program as it needs to. That said, the downside is that it then becomes very difficult to reason about the performance characteristics of your library, because its performance depends on how it's used. Simon Peyton-Jones is on record as saying that laziness is probably the wrong default for a language - the big benefit for Haskell was that it "kept them honest" wrt purity, but in a production system you probably want strictness.

evincarofautumn 13y ago

You can. A lazy algorithm will do no more reductions than an eager one, and it will usually do fewer. You just have to specify strictness. The compiler is pretty conservative about adding strictness because it has to preserve behaviour. But if you know something will be computed, e.g., a loop accumulator, then you can just tell the compiler so.