[Haskell-cafe] Re: quoting in Haskell
Peter Verswyvelen wrote: 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. I can't find something similar for Haskell? Maybe I am looking at the wrong places? Quoting/Inspecting code at runtime is not possible in Haskell since this would break referential transparency, i.e. one could tell that two extensionally equal values like 3 `add` 4 1 `add` (2 `mul` 3) are different by inspecting their internal structure. Of course, you can use constructors for add and mul and then inspect and transform the result data Expr = Val Int | Add Expr Expr | Mul Expr Expr add, mul :: Expr - Expr - Expr add = Add mul = Mul x :: Expr x = Val 1 `add` (Val 2 `mul` Val 3) By making Expr an instance of the class Num , you can use overloaded arithmetic operations instance Num Expr where (+) = Add (*) = Mul fromInteger = Val . fromInteger x :: Expr x = 1 + 2*3 I want to write something like selectiveQuote [add] (1 `add` 2 `mul` 3) which would result in an expression tree like add / \ 16 So the `mul` is not quoted because it is not part of the context = [add] I'm not sure why you'd want to do that, but it's not well-defined. What would selectiveQuote [add] ((1 `add` 2) `mul` 3) be? How to expand `mul` here when `add` isn't expanded? Regards, apfelmus ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: quoting in Haskell
Peter Verswyvelen [EMAIL PROTECTED] schrieb im Newsbeitrag news:[EMAIL PROTECTED] 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) The normal way of doing such things in Haskell is to have 1) functions that generate the component data structures (these functions are often called smart constructors) 2) other functions to put the functions/data structures together (these other functions are often call combinators). The resulting data structure that represents the sql query for example is then processed to produce the real (textual) sql query which this then sent to the database. I can't find something similar for Haskell? Maybe I am looking at the wrong places? HaskellDB for example does this for database queries. Parsec does this parsers. HSXML (if I got the name right) does this for XML. 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? A data constructor is a special case of a function, or perhaps better said, a particular way a function is defined. Either a function is a data constructor or it isn't. For example you can also do just = Just Just is a data constuctor. It was defined with a data statement (and as a result starts with a capital letter). data Maybe a = Nothing | Just a just is not a data constructor. Why? It wasn't defined with a data statement. However just and Just behave almost identically. (you can't pattern match on just, only on Just) Rene. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
