refactor(jsontools): restructure and simplify logic

plugins/jsontools/jsontools.plugin.zsh

 # JSON Tools
 # Adds command line aliases useful for dealing with JSON
 
-if [[ $(whence $JSONTOOLS_METHOD) = "" ]]; then
-	JSONTOOLS_METHOD=""
+# Check that user-defined method is installed
+if [[ -n "$JSONTOOLS_METHOD" ]]; then
+  (( $+commands[$JSONTOOLS_METHOD] )) || unset JSONTOOLS_METHOD
 fi
 
-if [[ $(whence node) != "" && ( "x$JSONTOOLS_METHOD" = "x"  || "x$JSONTOOLS_METHOD" = "xnode" ) ]]; then
-	alias pp_json='xargs -0 node -e "console.log(JSON.stringify(JSON.parse(process.argv[1]), null, 4));"'
-	alias is_json='xargs -0 node -e "try {json = JSON.parse(process.argv[1]);} catch (e) { console.log(false); json = null; } if(json) { console.log(true); }"'
-	alias urlencode_json='xargs -0 node -e "console.log(encodeURIComponent(process.argv[1]))"'
-	alias urldecode_json='xargs -0 node -e "console.log(decodeURIComponent(process.argv[1]))"'
-elif [[ $(whence python) != "" && ( "x$JSONTOOLS_METHOD" = "x" || "x$JSONTOOLS_METHOD" = "xpython" ) ]]; then
-	alias pp_json='python -c "import sys; del sys.path[0]; import runpy; runpy._run_module_as_main(\"json.tool\")"'
-	alias is_json='python -c "
+# If method undefined, find the first one that is installed
+if [[ ! -v JSONTOOLS_METHOD ]]; then
+  for JSONTOOLS_METHOD in node python ruby; do
+    # If method found, break out of loop
+    (( $+commands[$JSONTOOLS_METHOD] )) && break
+    # Otherwise unset the variable
+    unset JSONTOOLS_METHOD
+  done
+
+  # If no methods were found, exit the plugin
+  [[ -v JSONTOOLS_METHOD ]] || return 1
+fi
+
+# Define json tools for each method
+case "$JSONTOOLS_METHOD" in
+  node)
+    alias pp_json='xargs -0 node -e "console.log(JSON.stringify(JSON.parse(process.argv[1]), null, 4));"'
+    alias is_json='xargs -0 node -e "try {json = JSON.parse(process.argv[1]);} catch (e) { console.log(false); json = null; } if(json) { console.log(true); }"'
+    alias urlencode_json='xargs -0 node -e "console.log(encodeURIComponent(process.argv[1]))"'
+    alias urldecode_json='xargs -0 node -e "console.log(decodeURIComponent(process.argv[1]))"'
+  ;;
+  python)
+    alias pp_json='python -c "import sys; del sys.path[0]; import runpy; runpy._run_module_as_main(\"json.tool\")"'
+    alias is_json='python -c "
 import sys; del sys.path[0];
 import json;
 try: 
-	json.loads(sys.stdin.read())
+    json.loads(sys.stdin.read())
 except ValueError, e: 
-	print False
+    print False
 else:
-	print True
+    print True
 sys.exit(0)"'
-	alias urlencode_json='python -c "
+    alias urlencode_json='python -c "
 import sys; del sys.path[0];
 import urllib, json;
 print urllib.quote_plus(sys.stdin.read())
 sys.exit(0)"'
-	alias urldecode_json='python -c "
+    alias urldecode_json='python -c "
 import sys; del sys.path[0];
 import urllib, json;
 print urllib.unquote_plus(sys.stdin.read())
 sys.exit(0)"'
-elif [[ $(whence ruby) != "" && ( "x$JSONTOOLS_METHOD" = "x" || "x$JSONTOOLS_METHOD" = "xruby" ) ]]; then
-	alias pp_json='ruby -e "require \"json\"; require \"yaml\"; puts JSON.parse(STDIN.read).to_yaml"'
-	alias is_json='ruby -e "require \"json\"; begin; JSON.parse(STDIN.read); puts true; rescue Exception => e; puts false; end"'
-	alias urlencode_json='ruby -e "require \"uri\"; puts URI.escape(STDIN.read)"'
-	alias urldecode_json='ruby -e "require \"uri\"; puts URI.unescape(STDIN.read)"'
-fi
+  ;;
+  ruby)
+    alias pp_json='ruby -e "require \"json\"; require \"yaml\"; puts JSON.parse(STDIN.read).to_yaml"'
+    alias is_json='ruby -e "require \"json\"; begin; JSON.parse(STDIN.read); puts true; rescue Exception => e; puts false; end"'
+    alias urlencode_json='ruby -e "require \"uri\"; puts URI.escape(STDIN.read)"'
+    alias urldecode_json='ruby -e "require \"uri\"; puts URI.unescape(STDIN.read)"'
+  ;;
+esac
 
 unset JSONTOOLS_METHOD