Neil Mitchell wrote:
Hi
(I need to find some way to automate making these trails :) )
Yes! We'd need such an automatic tool for the wikibook, too.
The problem is that Haskell is ridiculously complex, and the "small step" interpretation is much harder than you'd think. For example, sum may well be defined as foldl' (+) 0, which is a CAF, so gets reduced once. The 0 won't actually be a 0, but will be fromInteger 0, which will correspond to looking up an item in the dictionary and applying it. Dictionaries especially make the "simple" interpretation completely wrong.
It's easy to do informally, but once you start being more precise, its very complex.
Like I said, I made a tool in Tcl that works. If you program in partially-parsed Haskell by hand first for all the functions it calls. (Except a few basic math ops.) Indeed, it was by playing with this tool that I first discovered why foldl' needs to exist! ;-) So, making a tool that you can set up to quickly generate an automated trace is quite easy. If you want a tool where you can just casually toss arbitrary Haskell at it and expect sensible answers... hmm... that's going to be kinda tricky. (!) (I had a go at it myself, several times. Each time I was tripped over by being unable to correctly parse arbitrary Haskell code. I never even got to writing the execution engine...!) I think a lot of people will agree that if such a tool existed it could be a *tremendous* help in many, many ways - a tool for experimenting and teaching, finding out why your really-complicated-function behaves wrong, checking out strictness properties, etc. But somebody has to write it first. It's ironic really; Haskell *looks* so easy to single-step. ;-)