encaps error when tag template returns object

[brigand]

This is the minimal:

function tag(strings) {
    return {};
}

var result = tag`foo`;

Gives:

/private/tmp/temp-27799/test.js:6:14,21: encaps tag
Error:
/private/tmp/temp-27799/test.js:3:12,13: object literal
This type is incompatible with
/private/tmp/temp-27799/test.js:6:14,21: string

Found 1 error

In the actual situation it's returning a well typed object.

The only way to avoid the error I've found is to declare the function's return type and the variable's type explicitly to any.

[samwgoldman]

Is it valid to return anything other than a string from a tagged template?

[samwgoldman]

If so, this should be pretty simple to fix. Tagged template expressions currently assume the tag function returns a string and the expression itself is a string. Reference Type_inference_js.ml.

[brigand]

Yeah it's just a CallExpression, the only special part is the arguments it receives.

The most permissive type for a function used as a template tag:

f(strings: (Array<string>) & ({raw: Array<string>}), ...args: any)

^{Edit: flow seems to not like that type definition}

Also strings and strings.raw are frozen, and strings[0] and strings.raw[0] are guaranteed to be strings.

[samwgoldman]

Thanks for the details! Unfortunately, array&object intersections don't currently do what you expect, but I think that's incompleteness, not intentional.

Flow also doesn't know what a frozen array is, for a similar issue (arrays are pretty different from objects in flow, but are objects in JS).

For now, though, the associated PR lets you return non-string values. I'll make sure we track those other two constraints in #560.