Consider the following grammar:

$$ A\to Ba|Aa|c \\ B\to Bb|Ab|d $$

How do I convert this grammar to be LL(1) by eliminating direct and indirect left recursion?

I have tried applying the rule which converts $A \to A\alpha|\beta$ to $A \to \beta A'$ and $A' \to \alpha A'$:

  • For $A \to Ba|Aa|c$, $\beta = Ba|c$ and $\alpha = a$.
  • For $B \to Bb|Ab|d$, $\beta = Ab|d$ and $\alpha = b$.

In this way I eliminate direct recursion from each one, but then when I try to substitute A or B with its product and eliminate indirect left recursion, the parsing table finds collisions so the grammar is not LL(1). Any ideas?


1 Answer 1


Mutual recusive definitions are tricky to eliminate. You'll have to work with the unwieldy expanded form of one of the rules. Here, let's expand $B$ \begin{align*} B &\to Bb \mid Ab \mid d \\ & \to (Bb \mid Ab \mid d)b \mid Ab \mid d \\ &\to ((Bb \mid Ab \mid d)b \mid Ab \mid d)b\mid Ab \mid d \\ & \to ~\cdots \\ &\to (d \mid Ab) \mid (db \mid Abb) \mid (dbb \mid Abbb) \mid \cdots \\ &\to (d \mid Ab)b^* \end{align*} In general, $B \to B(x_0 \mid x_1 \mid \dots) \mid (y_0 \mid y_1 \mid \dots)$ will have an expanded form of $B \to y\cdot x^*$. Things get even more complicated if $B \to z B x \mid y$ and $z$ is nullable. But let's not worry about that just yet, instead, let's turn our attention back to $B$.

Now, notice that I have used the term $b^*$, this is actually just the nonterminal $b^* \to \epsilon \mid b b^*$; this is a standard trick that many "extended" BNF based parsers use as well.

Finally, we can inline $B$ into $A$ to get \begin{align*} A &\to Ba \mid Aa \mid c \\ &\to db^* a \mid Abb^*a \mid Aa \mid c \\ &\to (c \mid db^*a)(bb^*a \mid a)^* \end{align*}

where once again $(bb^*a \mid a)^* \to \epsilon \mid (bb^*a \mid a)(bb^*a \mid a)^*$.

Now, if all you need is $A$, then this is already fine and dandy. However, if you need to expose $B$ as a public method as well, then you'll notice that there is a slight hitch: $\text{follow}((bb^*a \mid a)^*) = \text{follow}(A) = b$, since $B \to Abb*$. But since $\text{first}((bb^*a \mid a)^*)$ also contains $b$, then we can't decide whether we should shift to $\epsilon$ or $bb^*a$ when we encounter a $b$. You might try to inline $A$, so that $$ B \to (d \mid (c \mid db^*a)(bb^*a \mid a)^*b)b^* $$ but you still have the same problem. Now you can reason that $(bb^*a \mid a)^*bb^* = b^*(ab^*)^*$, but this is not very generalizable, and it breaks down fast when your language takes a step out of its regular territory. Instead, you'll want to start from the beginning. Expand $A$ first, and inline into $B$.

  • $\begingroup$ Τhe truth is i didn't understand everything, and this is supposed to be an undergraduate homework,is it possible that the answer is so complex ? I mean none of the examples we have been taught had something that complex $\endgroup$ Dec 6, 2016 at 18:09
  • $\begingroup$ but anyways thanks a lot for the answer !! :D $\endgroup$ Dec 6, 2016 at 18:10

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