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docs about 'when' statement

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Irmen de Jong 2019-07-10 00:45:53 +02:00
parent 14cabde5cf
commit a85743f241
2 changed files with 39 additions and 28 deletions
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25
docs/source/programming.rst
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@ -416,15 +416,26 @@ So ``if_cc goto target`` will directly translate into the single CPU instruction
Maybe in the future this will be a separate nested scope, but for now, that is
only possible when defining a subroutine.
when statement (jumptable)
^^^^^^^^^^^^^^^^^^^^^^^^^^
when statement ('jump table')
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. attention::
TODO: docs for this this must still be written.
TODO: the code generator for this is not yet working.
Instead of writing a bunch of sequential if-elseif statements, it is more readable to
use a ``when`` statement. (It will also result in greatly improved assembly code generation)
Use a ``when`` statement if you have a set of fixed choices that each should result in a certain
action. It is possible to combine several choices to result in the same action::
Use a ``when`` statement if you have a set of choices that each should result in a certain
action. It's more readable (and results in faster code) than using a lot of if / else statements.
when value {
4 -> c64scr.print("four")
5 -> c64scr.print("five")
10,20,30 -> {
c64scr.print("ten or twenty or thirty")
}
else -> c64scr.print("don't know")
}
The when-*value* can be any expression but the choice values have to evaluate to
compile-time constant integers (bytes or words). They also have to be the same
datatype as the when-value, otherwise no efficient comparison can be done.
Assignments

42
docs/source/syntaxreference.rst
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@ -610,28 +610,28 @@ The XX corresponds to one of the eigth branching instructions so the possibiliti
``if_cs``, ``if_cc``, ``if_eq``, ``if_ne``, ``if_pl``, ``if_mi``, ``if_vs`` and ``if_vc``.
It can also be one of the four aliases that are easier to read: ``if_z``, ``if_nz``, ``if_pos`` and ``if_neg``.
when statement (jumptable)
^^^^^^^^^^^^^^^^^^^^^^^^^^
.. attention::
TODO: docs for this this must still be written.
TODO: the code generator for this is not yet working.
when statement ('jump table')
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The structure of a when statement is like this::
The condition value can only be an integer datatype.
The choice values must be constant integer values.
when <expression> {
<value(s)> -> <statement(s)>
<value(s)> -> <statement(s)>
...
[ else -> <statement(s)> ]
}
code example::
The when-*value* can be any expression but the choice values have to evaluate to
compile-time constant integers (bytes or words).
The else part is optional.
Choices can result in a single statement or a block of multiple statements in which
case you have to use { } to enclose them::
when 4+A+Y {
10 -> {
c64scr.print("ten")
}
5 -> c64scr.print("five")
30 -> c64scr.print("thirty")
99 -> c64scr.print("nn")
55 -> {
; will be optimized away
}
else -> {
c64scr.print("!??!\n")
}
when value {
4 -> c64scr.print("four")
5 -> c64scr.print("five")
10,20,30 -> {
c64scr.print("ten or twenty or thirty")
}
else -> c64scr.print("don't know")
}