Code<?> Escaped As Statement

A few days ago, I changed the Mint compiler to allow non-void code objects to be spliced in where statement form is required:

Code x = <| 1 |>;
Code c = <| { `x } |>;

This essentially generates the code below, which allows the 1 to be discarded:

<| { { Object temp = 1; } } |>;

Eddy pointed out that there is still a problem: Since I do a static type check on the type argument of Code<T>, the proper form cannot be determined if there is a wildcard Code<?>. We can check at run-time whether a code object is in statement form or whether it is an expression whose result needs to be discarded, but we cannot simply write an if-statement, because one of the alternatives would always be syntactically incorrect:

<| { if (c.isStat) { `c; // already in statement form } else { Object temp = `c; // expression, discard result } } |>;

If c.isStat is true, then the else-branch is syntactically incorrect; if c.isStat is false, then there is a syntax error in the then-branch. Eddy pointed out that we can handle the Code<?> case, though, because the two branches are in future-stage code. That is, we can turn

<| { `(c); } |>


if (c.isStat) {
<| { `(c); } |>
} else {
<| Object temp = `c; |>

Unfortunately, this gets more complicated if there is more than one Code<?> being escaped in. We would have 2^n cases, where n is the number of Code<?> objects being stitched in:

<| { `(c); `(d); } |>

would have to be turned into

if (c.isStat && d.isStat) {
<| { `(c); `(d); } |>
} else if (c.isStat && !d.isStat) {
<| { `(c); { Object temp = `d; } } |>
} else if (!c.isStat && d.isStat) {
<| { { Object temp = `c; } `(d); } |>
} else {
<| { Object temp = `c; } { Object temp = `d; } |>

I think for now I will change the compiler so that escaping a Code<?> in as a statement generates a compiler error. It seems like this is an unimportant corner case, and I don’t want to spend my time on something that is rarely beneficial. Again, this only happens for Code<?>, and only when escaping into bracket with a code block where statement form is required by Java rules.


About Mathias

Software development engineer. Principal developer of DrJava. Recent Ph.D. graduate from the Department of Computer Science at Rice University.
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