* Carter Schonwald [2013-06-30 03:26:22-0400]

Otoh, would there be any ambiguity wrt applying functions to blocks?

eg f = (+ 1) h= f {let x = 7 in 3*x}, would that trip up the syntax?

This is not valid Haskell anyway (there's no such thing as "applying functions to blocks"). You can write h = f (let {x = 7} in 3*x) or h = f $ let {x = 7} in 3*x Roman

On Sun, Jun 30, 2013 at 2:59 AM, Judah Jacobson wrote:

...

Hi all,

I had a quick idea about record field syntax as specified in the GSoC project plan: http://hackage.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields/Plan .

Instead of "f.x" (to access field x of record f), maybe we could write "f{x}" as the record selection. That is, we'd reuse the brace notation which is already in place for record updates. Unlike dot notation, this is unambiguous and doesn't conflict with any existing syntax (AFAIK). It would also generalize to "f{x}{y}{z}" instead of "f.x.y.z".

This proposal would add two new forms of expressions:

f{x} to access field x of data f ({x}) = \f -> f{x} as a field access section

Additionally, it seems like record mutation expressions could also have sections:

({x=2}) = \f -> f{x=2}

That actually seems useful by itself, regardless of whether we use dot notation for field access.

Best, -Judah

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As long as we're bikeshedding... Possibly '#' is unused syntax -- Erlang uses it for its records too, so we wouldn't be pulling it out of thin air. E.g. "person#firstName" Tom El Jun 30, 2013, a las 22:59, AntC escribió:

...

Carter Schonwald writes:

indeed, this relates / augments record puns syntax already in GHC http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax- extns.html#record-puns.

Uh-oh. That documentation gives an example, and it exactly explains the weird type-level error I got when I tried to use the proposed syntax myself:

Note that:

* Record punning can also be used in an expression, writing, for example,

let a = 1 in C {a} -- !!!

instead of

let a = 1 in C {a = a}

The expansion is purely syntactic, so the expanded right-hand side expression refers to the nearest enclosing variable that is spelled the same as the field name.

IOW the proposal _does_ conflict with existing syntax. (And I guess I can see a use for the example. Note that outside of that let binding, `a` would be a field selector function generated from the data decl in which field `a` appears -- that's the weirdity I got.)

I suppose the existing syntax has a data constructor in front of the braces, whereas the proposal wants a term. But of course a data constructor is a term.

So the proposal would be a breaking change. Rats! Is anybody using that feature?

...

On Sun, Jun 30, 2013 at 2:59 AM, Judah Jacobson

gmail.com> wrote:

...

Unlike dot notation, this is unambiguous and doesn't conflict with any

existing syntax (AFAIK). ...

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If you really want to hunt for unused syntax and we wind up needing a (.) analogue then (->) is currently a reserved operator, so opening it up for use at the term level could be made to work, and there is a precedent with c/c++ pointer dereferencing. -Edward On Mon, Jul 1, 2013 at 1:10 AM, Edward Kmett wrote:

(#) is a legal operator today and is used in a number of libraries.

On Sun, Jun 30, 2013 at 11:38 PM, wrote:

...

As long as we're bikeshedding...

Possibly '#' is unused syntax -- Erlang uses it for its records too, so we wouldn't be pulling it out of thin air. E.g. "person#firstName"

Tom

El Jun 30, 2013, a las 22:59, AntC escribió:

...

...

Carter Schonwald writes:

indeed, this relates / augments record puns syntax already in GHC http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax- extns.html#record-puns.

Uh-oh. That documentation gives an example, and it exactly explains the weird type-level error I got when I tried to use the proposed syntax myself:

Note that:

* Record punning can also be used in an expression, writing, for example,

let a = 1 in C {a} -- !!!

instead of

let a = 1 in C {a = a}

The expansion is purely syntactic, so the expanded right-hand side expression refers to the nearest enclosing variable that is spelled the same as the field name.

IOW the proposal _does_ conflict with existing syntax. (And I guess I can see a use for the example. Note that outside of that let binding, `a` would be a field selector function generated from the data decl in which field `a` appears -- that's the weirdity I got.)

I suppose the existing syntax has a data constructor in front of the braces, whereas the proposal wants a term. But of course a data constructor is a term.

So the proposal would be a breaking change. Rats! Is anybody using that feature?

...

On Sun, Jun 30, 2013 at 2:59 AM, Judah Jacobson

gmail.com> wrote:

...

Unlike dot notation, this is unambiguous and doesn't conflict with any

existing syntax (AFAIK). ...

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Sure. I'd rather have nothing, but at least unlike the (.) proposals it doesn't break existing code. That said I don't think we need either. On Jul 1, 2013, at 2:27 AM, "John Wiegley" wrote:

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...

...

...

...

Edward Kmett writes:

...

If you really want to hunt for unused syntax and we wind up needing a (.) analogue then (->) is currently a reserved operator, so opening it up for use at the term level could be made to work, and there is a precedent with c/c++ pointer dereferencing.

Imagine this possible code:

foo :: Maybe Foo -> Bar foo (fromMaybe def -> x) = \x -> case x of Foo x -> x->y->z

I think it might get a bit ugly to give it a 5th meaning.

-- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net

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