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+date = "2013-03-01T00:00:00+01:00"
+title = "Xerl: expression separator"
+
++++
+
+As promised we are adding an expression separator this time.
+This will be short and easy.
+
+In the tokenizer we only need to add a line recognizing the
+comma as a valid token.
+
+[source,erlang]
+, : {token, {',', TokenLine}}.
+
+Then we need to change the following lines in the parser:
+
+[source,erlang]
+exprs -> expr : ['$1'].
+exprs -> expr exprs : ['$1' | '$2'].
+
+And add a comma between the expressions on the second line:
+
+[source,erlang]
+exprs -> expr : ['$1'].
+exprs -> expr ',' exprs : ['$1' | '$3'].
+
+That takes care of everything except the optional trailing
+comma at the end of our lists of expressions. We just need an
+additional rule to take care of this.
+
+[source,erlang]
+exprs -> expr ',' : ['$1'].
+
+That's it.
+
+Wondering why we don't have this optional trailing comma in
+Erlang considering how easy it was and the number of people
+complaining about it? Yeah, me too. But that's for someone else
+to answer.
+
+Another change I want to talk about is a simple modification
+of the compiler code to use an `#env{}` record for
+tracking state instead of passing around individual variables.
+This will be required later on when we make modules into proper
+expressions so I thought it was a good idea to anticipate.
+
+* https://github.com/extend/xerl/blob/0.4/[View the source]