Not to pour too much cold water on the idea of having string
interpolation literal, but I’d like to mention a few points here.
First, it was a deliberate design goal of the string template feature to
make interpolation an explicit act. Note that, if we had the syntax you
describe, we actually achieve the opposite effect: string interpolation
is now the default, and implicit, and actually /cheaper/ (to type) than
the safer template alternative. This is a bit of a red herring, I think.
The second problem is that interpolation literals can sometimes be
deceiving. Consider this example:
|String.format("Hello, my name is %s{name}"); // can you spot the bug? |
Where |String::format| has a new overload which accepts a StringTemplate.
Basically, since here we forgot the leading “$” (or whatever char that
is), the whole thing is just a big interpolation. Semantically
equivalent to:
|String.format("Hello, my name is %s" + name); // whoops! |
This will fail, as |String::format| will be waiting for an argument (a
string), but none is provided. So:
|| Exception java.util.MissingFormatArgumentException: Format specifier
'%s' | at Formatter.format (Formatter.java:2672) | at Formatter.format
(Formatter.java:2609) | at String.format (String.java:2897) | at (#2:1) |
This is a very odd (and new!) failure mode, that I’m sure is gonna
surprise developers.
Maurizio
On 14/03/2024 15:08, Guy Steele wrote:
Second thoughts about how to explain a string interpolation literal:
On Mar 13, 2024, at 2:02 PM, Guy Steele<[email protected]> wrote:
. . .
—————————
String is not a subtype of StringTemplate; they are disjoint types.
$”foo” is a (trivial) string template literal
“foo” is a string literal
$”Hello, \{x}” is a (nontrivial) string template literal
“Hello, \{x}” is a shorthand (expanded by the compiler) for
`String.of($“Hello, \{x}”)`
—————————
Given that the intent is that String.of (or whatever we want to call
it—possibly the `interpolation` instance method of class `StringTemplate`
rather than a static method `String.of`) should just do standard string
concatenation, we might be better off just saying that a string interpolation
literal is expanded by the compiler into uses of “+”; for example,
“Hello, \{x}.”
(I have added a period to the example to make the point clearer) is expanded
into
“Hello, “ + x + “.”
and in general
“c0\{e1}c1\{e2}c2…\{en}cn”
(where each ck is a possibly empty sequence of string characters and each ek is
an expression) is expanded into
“c0” + (e1) + “c1” + (e2) + “c2” + … + (en) + “cn”
The point is that, with this definition, “c0\{e1}c1\{e2}c2…\{en}cn” is a
constant expression iff every ek is a constant expression. This is handy for
interpolating constant variables into a string that is itself intended to be
constant.
—Guy
