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copied!<p>All this talk about recursive function and type is a red-herring. Scala's grammar does not allow for postfix operators in any other place than the end of an expression. This is the grammar we are talking about: the syntax of things without any semantics. Here is the relevant grammar from the specs:</p> <pre><code>Expr ::= (Bindings | [‘implicit’] id | ‘_’) ‘=>’ Expr | Expr1 Expr1 ::= ‘if’ ‘(’ Expr ‘)’ {nl} Expr [[semi] else Expr] | ‘while’ ‘(’ Expr ‘)’ {nl} Expr | ‘try’ ‘{’ Block ‘}’ [‘catch’ ‘{’ CaseClauses ‘}’] [‘finally’ Expr] | ‘do’ Expr [semi] ‘while’ ‘(’ Expr ’)’ | ‘for’ (‘(’ Enumerators ‘)’ | ‘{’ Enumerators ‘}’) | {nl} [‘yield’] Expr | ‘throw’ Expr | ‘return’ [Expr] | [SimpleExpr ‘.’] id ‘=’ Expr | SimpleExpr1 ArgumentExprs ‘=’ Expr | PostfixExpr | PostfixExpr Ascription | PostfixExpr ‘match’ ‘{’ CaseClauses ‘}’ PostfixExpr ::= InfixExpr [id [nl]] </code></pre> <p>The only two places where <code>PostfixExpr</code> appears beside these are after the <code>if</code> on a <code>case</code> statement and before <code>: _*</code> on an argument list. So, looking at that, we see that the only things that can appear on the right side of a postfix expression's method name is a type ascription or a <code>match</code>.</p> <p>So, what end expressions? Well, expressions appears in a lot of places in the grammar, so there's a lot of things that could end it. In this particular example, the expression is a <code>BlockStat</code> inside a <code>Block</code>, so it must end with a semi-colon, which may be inferred or not.</p> <p>To infer this semi-colon, it is necessary that the next line must not be something that could be parsed as another kind of expression. In this particular case, we have this:</p> <pre><code> val aFact = a! println("factorial of " + a + " is " + aFact) </code></pre> <p>Now, let's rewrite that from the point of view of the compiler:</p> <pre><code> val id = id id id ( stringLit id id id stringLit id id ) </code></pre> <p>These literals and identifiers are parsed like this:</p> <pre><code> val id = id id id ( expr ) val Pattern2 = SimpleExpr1 id SimpleExpr1 ArgumentExprs val Pattern2 = InfixExpr val Pattern2 = Expr val PatDef PatVarDef Def BlockStat </code></pre> <p>So that looks like a valid infix expression to the compiler as he parsed your program. Afterwards, it noticed the types didn't match, but it's too late to go back and see if a semi-colon could be inferred.</p>

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