"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
