Comments on: Thinking About Haskell*: You Know Lazy Evaluation; You Just Don’t Know It

By: alson

alson — Wed, 17 Dec 2008 05:49:53 +0000

Cale,

"That'll teach me to put in code I haven't tested in ghci..." Take 2! Fixed.

This little post just keeps on giving...

Alson

By: Cale

Cale — Wed, 17 Dec 2008 03:25:53 +0000

Just a note: $! is unfortunately not left associative, but right associative, so that last example won't work, you have to write

f a b c = ((g $! a) $! b) $! c

(also, you had the f being recursive there, which probably wasn't what you'd intended)

I'd like to change $ and $! to be left associative like function application normally is for this, and a few other reasons.

By: alson

alson — Tue, 16 Dec 2008 17:08:29 +0000

Andreas,

I tried to make note of that, but didn't do a great job. Adding more words...

Alson

By: alson

alson — Tue, 16 Dec 2008 16:52:22 +0000

Luke, Josef, Asger,

Agreed. That'll teach me to put in code that I haven't tested in ghci... Fixed.

Alson

By: alson

alson — Tue, 16 Dec 2008 16:49:54 +0000

Worthless,

In terms of "Thinking about Haskell", seeing '&&' as lazy has helped me think about Haskell. Laziness can be pretty rough to grapple with and I thought that people would appreciate the '&&' example as a lazy-ish thing they've used lots of time.

Besides, I'm a little confused by your comment that '&&' isn't lazy. From the HaskellWiki:

"Lazy evaluation causes that expressions are not evaluated when they are bound to variables, but their evaluation is deferred until their results are needed by other computations."

'&&', whether in C# or Haskell, seems to fit that description.

Alson

By: alson

alson — Tue, 16 Dec 2008 16:42:43 +0000

Roman, Argh! Fixed. – Alson

By: worthless

worthless — Tue, 16 Dec 2008 14:06:23 +0000

GHCi> case undefined of _ -> "test" "test"

...

By: worthless

worthless — Tue, 16 Dec 2008 14:05:42 +0000

“if ( (obj != null) && (obj.someMethod() == somethingElse) ) {” This is not lazy evaluation, this is sequential and conditional execution.

By: augustss

augustss — Tue, 16 Dec 2008 13:44:09 +0000

Actually, using case like that doesn’t evaluate anything. The case expression evaluates exactly as much as it has to make sure the pattern matches. And since _ matches everything then nothing is evaluated. You need to use the (non-lambda-definable) seq function to evaluate arbitrary values.

By: Josef Svenningsson

Josef Svenningsson — Tue, 16 Dec 2008 13:20:06 +0000

Your last example is wrong. A case that doesn’t use it’s result will not force the thunk being cased. Use bang patterns or deepSeq instead.