[Haskell-beginners] About ! before a type signature

Kim-Ee Yeoh ky3 at atamo.com
Sat Jun 7 05:15:07 UTC 2014

On Sat, Jun 7, 2014 at 11:14 AM, Xiaojun "Phil" Hu <phil at cnphil.com> wrote:

> I found it relatively hard to reason about the time complexity of Haskell
> programs.

Is that the question you really want to ask?

If it is, the answer is trivial: if it's O(f(n)) time in other languages,
it's also O(f(n)) time in Haskell. (Note: space is a different story.)

But what typically people want to know is: why is my program so slow?

You wonder about the constants covered up by Big-Oh.

And so you're going to have to get acquainted with

0) lambda calculus
1) graph reduction,
2) the G-machine, and
3) the spineless-tagless variant thereof.

You're also going to have to learn to read Core and understand some of the
higher order core-to-core optimizations that GHC performs.

-- Kim-Ee
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