On Tue, Nov 30, 2010 at 05:50:02PM -0200, José Romildo Malaquias wrote:
Hello.
When learning how to use the many regular expression libraries for Haskell, I noticed that the interface API from the regex-base package introduces several high level operations that are abstracted from the implementations (backends). This is done by means of classes.
For instance, there are methods for compiling an external representation (like a string or a bytestring) into a regular expression. There are also methods for matching a regular expression and some text (like a string, or a bytestring).
But the most high level functions for pattern matching using regular expressions, (=~) and (=~~), are not defined as methods in a class. They are independent functions defined for each backend in the corresponding package.
This prevents one from writing a general function using these operators without deliberately choosing a regex backend.
Why are those operators not defined as methods, like all other relevant functions?
I think I have found the reason for that. Let's consider the operator (=~ ). Its type in the Text.Regex.TDFA is: ( RegexMaker Regex CompOption ExecOption source , RegexContext Regex source1 target ) => source1 -> source -> target It has two arguments: the text and the regex (in a textual format) used in matching. They may be both of type String, or ByteString, for instance. Therefore there is no way to infer which regex engine to use when the regex text is compiled to an internal format. So the regex engine cannot be deduced automatically from the arguments. Because of that each regex backend has to define his own function, and a class is of no help. Romildo