-z4h-is-valid-list

#!/usr/bin/env zsh
#
# Succeeds if $1 is a well-formed command.

# If it ends with an odd number of backslashes, it's malformed.
() {
  builtin emulate -L zsh -o extended_glob
  [[ $1 == (|*[^\\])(\\\\)#\\ ]]
} "$1" && builtin return 1

if [[ -v functions[-z4h-test-func] ]]; then
  builtin unfunction -- -z4h-test-func
fi
functions[-z4h-test-func]="$1" 2>/dev/null              || builtin return 1
[[ -v functions[-z4h-test-func] ]]                      || builtin return 1
builtin unfunction -- -z4h-test-func
# This suffix allows us to detect two tricky cases: "for x" and "<<END".
# Each of these on their own is a valid function body but neither can
# be executed as a command in interactive shell.
functions[-z4h-test-func]="$1"$'\ndo\ndone' 2>/dev/null || builtin return 0
[[ -v functions[-z4h-test-func] ]]                      || builtin return 0
builtin unfunction -- -z4h-test-func
builtin return 1