Feather::Curry

Last updated: 8th July 1999 at 10:49 BST

This package defines a new command object type "feather::curried" and a command "feather::curry" to create objects of that type.

`[feather::curry]`

Syntax

feather::curry <command> ?<args> ...?

Creates a curried object and returns it. The command argument specifies the command to invoke when the curried object is invoked and the args are the arguments to pass to command.

Currying of curried objects is optimised to improve the performance. Rather than embed curried objects inside one another they are expanded out.

Description

Currying creates a command which takes N-k arguments by encapsulating a command which takes N arguments with k arguments. This is similar to how [list] is used to build up callback scripts.

fileevent $channel readable [list AsyncRead $channel]
button .b$i -label "Button $i" -command [list InvokeButton $i]

The curried object has the following advantages over the list approach.

It can be used as a command and does not need to be passed through [eval].
The internal representation of its objects are guaranteed to be preserved, even when passed through [eval].
The above also mean that it is faster (and more scaleable).

Examples

Creating an object command is very easy. Obviously as you have no control over the name that the object gets this will not work when the name is important, e.g. for widgets.


namespace eval objects {

    proc ctor {args} {
	# Create and initialise some instance data.
	# $self 'points' to the instance data.

	return [feather::curry [namespace current]::methods $self]
    }

    proc methods {self method args} {
        # Use $self to access the instance data.

	switch -- $method {
	    "print" {
	        puts "$args"
	    }
	}
    }
}

set object [objects::ctor x y z]
$object print 2 3

Doing functional programming.

proc square {x} {
    expr {$x * $x}
}

proc composer {f g x} {
    $f [$g $x]
}

proc compose {f g} {
    # returns a function equivalent to [$f [$g $x]]
    return [feather::curry composer $f $g]
}

set quad [compose square square]

Object type: feather::curried

It supports the following interfaces.

command