Re: [Haskell-cafe] Transformation sequence

Hi Alfonso & Andrew Alfonso Acosta wrote:

How about using a state monad as a logger?

I am not a monad-expect, so I may be wrong, but wouldn't a writer monad be more appropriate? After all, it is just used for logging the intermediate results, not to keep read/write state. In other words, we just need to read the "logged" values when the transformation has occurred, not while it is occurring. Greetings, Mads Lindstrøm

You store the transformation sequence in the state while processing the tree, then you simply retrieve the state and print it out.

Your transformation function should change to

...

import Control.Monad.State

data Log = ... -- to be defined

type LogMonad a = State Log a

transform :: LogMonad Expression -> LogMonad [Expression]

On 10/20/07, Andrew Coppin wrote:

...

I'm writing some code where I take an expression tree and transform it into another equivilent one.

Now it's moderately easy to write the code that does the transformation. But what I *really* want is to print out the transformation *sequence*. This appears to be much more awkward.

What I have is a function like this:

transform :: Expression -> [Expression]

The trouble is, if you want to apply the transformation recursively, things get very messy. Oh, it *works* and everything. It's just really messy and verbose. In Haskell, this is usually a sign that you want to start applying some ingenious trickery... but I'm having an ingeniety failure here.

Suppose, for example, that in one case you want to recursively transform two subexpressions. I end up writing something like

transform (...sub1...sub2...) = let sub1s = transform sub1 sub2s = transform sub2 in map (\sub1' -> put sub1' back into main expression) sub1s ++ map (\sub2' -> put sub2' back into main expression) sub2s

After you've typed that a few times, it becomes *very* boring! But I can't think of a clean way to abstract it. :-(

It's *almost* like you want to use the list monad:

transform (...sub1...sub2...) = do sub1' <- transform sub1 sub2' <- transform sub2 return (put sub1' and sub2' back into the main expression)

Except that that doesn't quite work properly. As shown above, I actually want to go through all the transformation steps for the first branch, and *then* all the steps for the second branch.

Any hints?

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