Syntax for polymorphic values.

[TomasMikula]

This is another attempt at convenient syntax for polymorphic values. This PR addresses #37 and supersedes #44.

The syntax

ν[T].m[A, B[_], ...](e)

is rewritten to

new T { def m[A, B[_], ...] = e }

(The symbol "ν" is the Greek lowercase letter "Nu" (/njuː/) and stands for new.)

In case m has just one *-kinded type parameter which is not referenced from e, it can be omitted:

ν[T].m(e)

is rewritten to

new T { def m[A] = e }

where A is a fresh name.

If the method name is apply, it can be omitted:

ν[T][A, B[_], ...](e)

is rewritten to

new T { def apply[A, B[_], ...] = e }

The previous two shortcuts can be combined:

ν[T](e)

is rewritten to

new T { def apply[A] = e }

where A is a fresh name.

There is also an alternative syntax using Λ (Greek uppercase letter Lambda)

Λ[A, B[_], ...](e): T

which is equivalent to

ν[T][A, B[_], ...](e)

i.e.

new T { def apply[A, B[_], ...] = e }

When using Λ-syntax, type parameters cannot be omitted, and method name is (currently) assumed to be apply. Note also that the type T of the whole Λ-expression has to be specified (cannot be inferred, since kind-projector runs before typer).

See the test cases in polyval.scala for some examples.

[TomasMikula]

I fixed the 2.10 build failure and made the new functionality opt-in via scalac option

-P:kind-projector:forall=true

[TomasMikula]

I changed the name from ∀ to ν (Greek lowercase letter Nu, pronounced "new"), since it better fits the order of arguments.

I also added alternative Λ-syntax which resembles System-F syntax for universal quantification.

I updated the description above to reflect these changes.

[TomasMikula]

I published this functionality as a separate plugin: P∀scal.

I consider polymorphic values (including existing polymorphic lambdas) a separate concern from kind-projector's type lambdas through type projections. Nevertheless, I'm leaving this open for a while in case people want to see this in kind-projector anyway.

[fommil]

can you support the plain English spellings too? In Emacs I have a prettify-greek mode which will render these things as UTF-8 but save as ASCII to disk. It also makes it a lot easier for me to type...

[TomasMikula]

I could do Nu, but Lambda in term position is already reserved by kind-projector for polymorphic lambdas.

For typing, I have just installed Greek keyboard layout.

[fommil]

I think it would need to be the lowercase versions for prettify-greek to work (it'll use capitals)

[TomasMikula]

For kind-projector, Lambda and λ are already synonyms. It would be confusing to make lambda and Λ synonyms.

[TomasMikula]

I'm going to close this for the following reasons:

1. With the addition of self-referencing polymorphic values, P∀ascal has by now diverged from this PR.
2. New features add maintenance cost, which I don't mean to incur on kind-projector.
3. kind-projector already claims the identifier λ from the name space. P∀scal claims another two, ν and Λ. Rather than having configuration options for what features of the plugin to turn on (and thus what identifiers will be reserved), the user may just add separate plugins à la carte.