Parameters

does this change affect me?

If you use Cylc parameters with negative offsets (e.g. foo<x-1>).

Overview

There has been a subtle change in the way negative offsets are handled in parameters.

Example

If you have a parameter x with the values 1, 2 & 3:

[task parameters]
   x = 1..3

And use it like so:

a<x-1> => b<x> => c<x>

There is some ambiguity about how this should be interpreted when x=1 because <x-1> would be 0 which is not a valid value for the parameter x.

Cylc 7 removed the part of the expression which was out of range resulting in a partial evaluation of that line:

        b_x1 => c_x1  # x=1
a_x1 => b_x2 => c_x2  # x=2
a_x2 => b_x3 => c_x3  # x=3

Whereas Cylc 8 will remove everything after the first out-of-range parameter - <x-1> (so the entire line for this example):

a_x1 => b_x2 => c_x2  # x=2
a_x2 => b_x2 => c_x2  # x=3

Migration

If your workflow depends on the Cylc 7 behaviour, then the solution is to break the expression into two parts which Cylc will then evaluate separately.

- a<x-1> => b<x> => c<x>
+ a<x-1> => b<x>
+ b<x> => c<x>

Resulting in:

# a<x-1> => b<x>
a_x1 => a_x2  # x=2
a_x2 => a_x3  # x=3

# b<x> => c<x>
b_x1 => c_x1  # x=1
b_x2 => c_x2  # x=2
b_x3 => c_x3  # x=3

Line Breaks

Note that these expressions are all equivalent:

a<x-1> => b<x> => c<x>

a<x-1> => b<x> => c<x>

a<x-1> => b<x> => \ c<x>