[ghc-steering-committee] Is @ a name-space override, or a visibility override?

[Spiwack, Arnaud]

Dear all,

There is a debate which has been held in our heads for quite some time now.
But we’ve never really had the conversation. This debate has been raised
again in the context of proposal #281. But I’d rather make a separate
thread about it.

The question is the following:

When I write

f @t

Am I using @ as a way to change an invisible argument into a visible
argument? Or am I using @ to insert a type expression into a term
expression?
------------------------------

The truth is that it’s doing both at the same time. So some of us have
taken to believing that the One True Meaning™ of @ is to override
(in)visibility. While others have taken to believing that the One True
Meaning™ of @ is to insert types in term.

And, in both cases, we believe the other part to be incidental.

In favour of types-in-term, there is the pretty-printer for Core, which, if
I’m not mistaken, uses @t to denote type applications (and all type
applications are visible in Core, so there is no visibility override to be
had).

Whatever we think about this, it appears that the fact that this meaning
ascription is implicit has caused some anguish. So I think that we should
have this discussion explicitly for once.

I chose to do so in a different thread than the #281 discussion, because
this discussion stands on its own.

Anyway, the floor is yours.

/Arnaud
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