How zsh zle prevent variable CURSOR to be modified arbitrarily in runtime? - zsh

Question

As the document says, the zle variable CURSOR can only be in range [0, $#BUFFER].
Test code (put it into .zshrc, the ^[OP is F1):
testCursor() {
echo "\nOriginal C: $CURSOR"
BUFFER="a"
echo "Change Buffer: $CURSOR"
CURSOR=$((CURSOR+10))
echo "Force edit: $CURSOR"
CURSOR=100
echo "Force assign: $CURSOR"
}
zle -N testCursor
bindkey '^[OP' testCursor
The CURSOR satisfied it's range definition in runtime, how did the zsh-zle implements it?

Answer 1

The CURSOR value is handled in Zsh's source code, which is implemented in the C programming language: https://github.com/zsh-users/zsh/blob/3c93497eb701d8f220bc32d38e1f12bfb534c390/Src/Zle/zle_params.c#L266
There is no way for you to declare a similarly constrained variable in Zsh shell code.
However, you can write a math function for it instead:
# Declare a global integer.
typeset -gi TEST=0
# -H makes these hidden, that is, not listed automatically.
typeset -gHi _TEST_MIN=0 _TEST_MAX=10
# Load `min` and `max` functions.
autoload -Uz zmathfunc && zmathfunc
set_test() {
(( TEST = min(max($1,$_TEST_MIN),$_TEST_MAX) ))
}
get_test() {
return $(( min(max($TEST,$_TEST_MIN),$_TEST_MAX) ))
}
# Declare `set_test` as a math function accepting exactly one numeric argument.
functions -M set_test 1
# Declare `get_test` as a math function accepting exactly zero arguments.
functions -M get_test 0
You can then use these in arithmetic statements, with this syntax:
❯ print $(( get_test() ))
0
❯ (( set_test(100) ))
❯ print $(( get_test() ))
10
But also in other contexts, with this syntax:
❯ set_test -1
❯ get_test; print $?
0