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    <p>$1, $2, and so on refer to the terms on the right-hand side of a grammar rule. For example in</p> <pre><code>stat: expr { printf("%d\n",$1); } | LETTER '=' expr { regs[$1] = $3; } </code></pre> <p>LETTER '=' expr is one of the rules and in the following parentheses $1 refers to LETTER. <code>regs[$1] = $3;</code> will be made into a C statement but in order to do that, yacc needs to know what type $1 has. If you add</p> <pre><code>%type &lt;a&gt; LETTER </code></pre> <p>after the first %type declaration (or simply list <code>LETTER</code> after expr) the first error will be taken care of. Same goes for <code>DIGIT</code> and <code>base</code>. Note that there is nothing that refers to the value of stat (naturally) so there is no need for a %type declaration for stat. Thus in</p> <pre><code>calc.yacc:105.17-18: $1 of `number' has no declared type calc.yacc:106.20-21: $1 of `number' has no declared type calc.yacc:110.29-30: $2 of `number' has no declared type </code></pre> <p>the first line implies that <code>DIGIT</code> has an unknown type, the second line refers to the same problem with <code>number</code>; finally the last line reminds you to declare the type for <code>base</code>. Here is the yacc code it is referring to:</p> <pre><code> number: DIGIT { $$ = $1; base = ($1==0) ? 8 : 10; } | number DIGIT { $$ = base * $1 + $2; } ; </code></pre> <p>Finally, without getting into too many details, the statement</p> <pre><code>regs[$1]=$3; </code></pre> <p>will be translated by yacc into something close to:</p> <pre><code>regs[YS[1].&lt;type of LETTER&gt;]=YS[3].&lt;type of expr&gt;; </code></pre> <p>where YS is a 'magic array' (actually yacc's stack); YS has the type of the declared %union. Thus you can see that to make this into legal C, yacc needs to know which member of the %union <code>&lt;type of LETTER&gt;</code> refers to. This is what the <code>%type</code> declaration is for.</p>
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    1. COIncidentally, all the errors would also go away if you removed the `%union` and `%type` directives. The default type for the stack is `int`, and without a `%union` yacc assumes all tokens and nonterminals have the same type (`YYSTYPE` if I recall correctly, default `int` anyway). Use `%union` and `%type` to set different types for different tokens/nonterminals.
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