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I am resuming syntax rules for a small language:

\begin{eqnarray*} e_C &::=& \epsilon \mid constant \\ \textit{prefix-op} &::=& - \\ \textit{infix-op} &::=& + \mid - \mid * \\ e_E &::=& e_C \mid \textit{prefix-op} \; e_E \mid e_E \; \textit{infix-op} \; e_E \mid \textit{function}_E (e_{E,0}, e_{E,1},\ldots) \mid \textit{specialE} \\ e_V &::=& e_C \mid \textit{prefix-op} \; e_V \mid e_V \; \textit{infix-op} \; e_V \mid \textit{function}_V (e_{V,0}, e_{V,1}, \ldots) \mid \textit{specialV} \end{eqnarray*}

The expressions $e_E$ and $e_V$ have something common: $e_C$. Some of their operators look same: $\textit{prefix-op}$ and $\textit{infix-op}$. Their functions $\textit{function}_E$ and $\textit{function}_V$ are 2 different sets, and $\textit{specialE}$ and $\textit{specialV}$ are totally different.

I am wondering if it is still possible to present this syntax more succinct, more compact in a conventional way... Could anyone help?

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2 Answers

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Van Wijngaarden grammars are a way to handle this problem. It would be something like:

KIND : e ; v .

e KIND : e c ; prefix-op, e KIND ; e KIND, infix-op, e e; function KIND (e KIND, ",", e KIND, "," ... ) ; special KIND .

(There is no "..." in the formalism, so you'd have either to extend the grammar syntax or express it in another way).

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One direct way of solving this problem is to make the syntax rules parameterised, as follows:

\begin{eqnarray*} e_{X,Y} &::=& e_C \mid \textit{prefix-op} \; e_{X,Y} \mid e_{X,Y} \; \textit{infix-op} \; e_{X,Y} \mid Y (e_{X,Y}, e_{X,Y},\ldots) \mid X \\ \end{eqnarray*}

And then to set \begin{eqnarray*} e_E &::=& E_{\mathit{specialE},~\mathit{function}_E} \\ e_V &::=& E_{\mathit{specialV},~\mathit{function}_V} \\ \end{eqnarray*}

This is using notation I just invented, and it could probably be reworked, but you get the idea, I hope.

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