Jeremy Gibbons <Jeremy.Gibbons@comlab.ox.ac.uk> wrote in article <Pine.GSO.4.51.0501310314250.3639@abacus.comlab> in gmane.comp.lang.haskell.general:
But in FP, of course, functions are first class citizens. So one may get ambiguities on account of it being reasonable to treat a particular function either as "program" or as "data" - with conflicting outcomes...
Suppose one wants to streamline notation, so that instead of having to write (f . g) $ x one can write f @ g @ x Here, "@" means either composition or application according to context. It is supposed to be associative, so it isn't the same as normal Haskell $; in particular, f @ g is valid and represents the composition of f and g.
Indeed, I've been thinking about such a notation, but one that explicitly specifies whether to treat a function as "program" or "data". Variables like "f" always denote data to start with; such data can be thought of as a program that maps unit to a function. Data can be turned into a program by unquoting it with "," (usually I write "@" for unquotation but you use it for composition already). (Conversely, a program can be turned into data by enclosing it in a pair of quotation brackets "[]", a la Joy; unquotation cancels quotation by the reduction step ,[X] => X.) Now (f . g) $ x in Haskell notation is written ,f @ ,g @ x We also distinguish between ,thrice @ thrice and ,thrice @ ,thrice An advantage of this notation is that it exposes the symmetry between call-by-value and call-by-name evaluation: right-to-left evaluation here corresponds to call-by-value, argument-first evaluation; left-to-right evaluation here corresponds to call-by-name evaluation. I could go on and on about this. -- Edit this signature at http://www.digitas.harvard.edu/cgi-bin/ken/sig War crimes will be prosecuted. War criminals will be punished. And it will be no defense to say, "I was just following orders."' George W. Bush, address to the nation, 2003-03-17 http://www.whitehouse.gov/news/releases/2003/03/20030317-7.html