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Re: [Haskell-cafe] trace

Subject: Re: [Haskell-cafe] trace
From: Daniel Fischer
Date: Thu, 24 Dec 2009 23:04:31 +0100

Am Donnerstag 24 Dezember 2009 22:21:50 schrieb michael rice:

> Hmm... here are the functions I was looking to trace, the second one being

> an example from Scheme text "Concrete Abstractions" that I rewrote after

> seeing the first. Compared to the CL/Scheme memoization code, the Haskell

> seems like, how shall I put this, a sleight of hand, so much so that I'm

> driven to look behind the scenes to try to understand what is occurring. I

> remember that someone said, pattern matching is strict and LET is lazy, so

> I know the trick depends on laziness, but knowing that and understanding it

> are still a world apart.

>

> Does tracing a function *always* require memoizing it?

No, of course not. That was just a use of tracing I knew where it was. You can put calls to trace more or less everywhere, like

mySine x = sin x `debug` "Calling sine " ++ show x

(note: using debug = flip trace instead of trace needs fewer parentheses and makes commenting out the tracing code easier).

Of course, tracing can be particularly enlightening for recursive functions.

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