On 8/27/07, Peter Verswyvelen
In Scheme, on can "quote" code, so that it becomes data. Microsoft's F# and C# 3.0 also have something similar that turns code into "expression trees". The latter is used extensively in LINQ which translates plain C# code into SQL code or any other code at runtime (this idea came from FP I heared)
I can't find something similar for Haskell? Maybe I am looking at the wrong places?
In Haskell, I know one can use a data constructor as a function (as in (map Just [1..3])), but a function cannot be turned into a data constructor (= "quoting"), can it?
Now this is all really fuzzy for a newbie like me, because aren't all functions initially just data constructors waiting to be evaluated in a lazy language?
I'm actually looking for something like (loose terminilogy follows) "context-based-semi-quoting". The idea is to only quote a set of functions, while evaluating all the others.
For example, in the code
1 `add` 2 `mul` 3 where add = (+) mul = (*)
I want to write something like
selectiveQuote [add] (1 `add` 2 `mul` 3)
which would result in an expression tree like
add / \ 1 6
So the `mul` is not quoted because it is not part of the "context" = [add]
Maybe this is just impossible, I did not dig deep into this.
Thanks, Peter
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Look at Template Haskell. quote from http://haskell.org/th : "Intuitively Template Haskell provides new language features that allow us to convert back and forth between concrete syntax, i.e. what you would type when you write normal Haskell code, and abstract syntax trees. These abstract syntax trees are represented using Haskell datatypes and, at compile time, they can be manipulated by Haskell code. This allows you to reify (convert from concrete syntax to an abstract syntax tree) some code, transform it and splice it back in (convert back again), or even to produce completely new code and splice that in, while the compiler is compiling your module." However I don't know if your 'selectiveQuote' is possible using TH. regards, Bas van Dijk