Towards a List Comprehension in Tcl
A very basic prototype implementation of a list comprehension for Tcl.
Why?
For fun, and because it's 4am and I'm trying to get the baby to sleep.
Usage
You just need to source listc.tcl and the listc command will be available in the global namespace.
Usage is described by the shell as:
listc ?expression? ?input_set? ?predicate? ?variable_name?
Here's an example to double all odd numbers between 1 and 10 inclusive:
% source listc.tcl
% listc 2*x [list 1 10] x%2==1
2 6 10 14 18
Observe how listc takes calculations in the format expected by the expr statement.
All arguments are optional. To play around with it, my recommendation would be to open listc_test.tcl and have a look at the examples within the unit tests. They show the list given the conventional way with the list command alongside the list comprehension given with listc.
Limitations
There are three main limitations of this prototype.
Firstly, the list is created at the time listc is called and is represented as a standard Tcl list. This means you can't specify infinity in the range as you can in languages such as Python. I think enabling this would require overriding the standard list commands to perform the computation when list items are requested.
Secondly, the evaluation of the expressions is not restricted in any way. I wouldn't run this in production without carefully sanitising any variables you substitute into your listc statement. Then again, the same rule of thumb would also apply to list anyway.
Finally, the variable substitution is pretty bad. I would just stick with x as the variable name, and not change it using the variable_name argument.