zsh: add function to prepend to PATH without duplicates

With this function, I can use the same logic in other configuration
files (e.g. `~/.local/etc/zsh/config.zsh`). This avoids duplication in
tmux sessions when prepending naively.

config/zsh/.zprofile

@@ -31,12 +31,30 @@ have() {
 #
 [ "$(uname -s)" = "Darwin" ] && { PATH=""; source /etc/profile; }
 
+# Prepend $2 to variable $1 using $3 as separator, but only if $2 is not already
+# contained in $1. This is useful for prepending to PATH while avoiding
+# duplicates.
+prepend_unique() {
+  local var="$1"
+  local new="$2"
+  local sep="${3-:}"  # Default separator is colon.
+  local old="${(P)var}"
+
+  case "${sep}${old}${sep}" in
+    *"${sep}${new}${sep}"*)  # Skip duplicate.
+      ;;
+    "${sep}${sep}")  # Variable was empty, set without separator.
+      typeset -g $var="${new}"
+      ;;
+    *) # Prepend with separator.
+      typeset -g $var="${new}${sep}${old}"
+      ;;
+  esac
+}
+
 # Add custom bin dirs to PATH if they exist and are not already in PATH.
 while read -r dir; do
-  [ -d "$dir" ] && case ":${PATH:=$dir}:" in
+  [ -d "$dir" ] && prepend_unique PATH "${dir}"
-    *:"$dir":*) ;;          # already in PATH -> ignore
-    *) PATH="$dir:$PATH" ;; # not yet in PATH -> prepend it
-  esac
 done <<EOL
   $LOCAL_PREFIX/bin
   $LOCAL_PREFIX/opt/curl/bin
@@ -56,10 +74,7 @@ export PATH
 # available.
 have manpath && MANPATH="$(unset MANPATH; manpath)"
 while read -r dir; do
-  [ -d "$dir" ] && case ":${MANPATH:=$dir}:" in
+  [ -d "$dir" ] && prepend_unique MANPATH "${dir}"
-    *:"$dir":*) ;;                # already in MANPATH -> ignore
-    *) MANPATH="$dir:$MANPATH" ;; # not yet in MANPATH -> prepend it
-  esac
 done <<EOL
   $LOCAL_PREFIX/share/man
   $LOCAL_PREFIX/opt/curl/share/man