Whether to have contexts where nullable annotations are not allowed

[jeffdyer]

Should there be contexts where nullable annotations are not allowed by the grammar? — Jeff Dyer 2006/12/15 08:38

   x to t?, x cast t?  // cast/to
    x is t?             // relational
    (t?).<u,v>          // type identifier
    {x:1}:t?            // object and array tags
    {x:x}:t? = o   // typed patterns

If we don’t allow ? then shouldn’t we disallow ! and make the type expression mean ! by default? Discussion in Wiki

[jeffdyer]

Resolution: we have two type expression non-terminals, NullableTypeExpression? and TypeExpression?.

TypeExpression? occurs in the following contexts:

• Annotating an object or array literal or pattern
• Right side of 'cast', 'is', and 'to'
• In a NullableTypeExpression?
• In a parameterised type instantitation of the form ( TypeExpression? ) .< ... >