attrs.interactions

Module nu.context.attrs.interactions.

Attr-fabric interactions: SetCmd, Delete, AttrExists, Let.

The write ops (SetCmd / Delete) delegate to the Ref (ref._write / ref._erase) so the write mechanism lives with the fabric, not hardcoded here. AttrExists complements the dual-role read: an unbound read yields EMPTY, which a bound EMPTY would alias, so existence needs an explicit query.

Let is a scoped attr binding: a Bracket that pushes name -> value into ctx.attrs for the body's duration and restores the prior slot on exit (also on exception). It lives here (not with the fabric-lifecycle brackets in context/fabric/lifecycle.py) because the binding it governs is a scratch attr, not a fabric instance - same axis as SetCmd / AttrRef, just scoped rather than open-ended.

Each interaction holds its Ref in a mutation or read slot; effect synthesis binds it to the right effect on the attrs fabric.

Name	Sort	Call	Meaning
AttrExists	`scalar_query`	`AttrExists(ref)`	Whether the address of its `AttrRef` is bound in `ctx.attrs`.
Delete	`scalar_command`	`Delete(ref)`	Removes the slot its Ref names from the fabric, through that Ref.
Let	`bracket`	`Let(name, value, body=None)`	Binds a name to a value in `ctx.attrs` for the body's duration.
SetCmd	`scalar_command`	`SetCmd(ref, value)`	Writes a value into the slot its Ref names, through that Ref.

AttrExists

Whether the address of its AttrRef is bound in ctx.attrs.

AttrExists(ref)

Path nu.context.AttrExists. Kind ScalarQuery, sort scalar_query, cardinality scalar. Arity 1 (1 required).

Arguments

Name	Type	Default	Meaning
`ref`			the `AttrRef` whose address is resolved and looked up.

Yields

True or False, never a sentinel. An address that resolves to EMPTY or INVALID is looked up as a key like any other, and is simply absent.

Notes

Normally written as AttrRef(...).exists() rather than built by hand.
Exists because the dual-role read cannot answer the question: an unbound slot yields EMPTY, and so does a slot holding EMPTY.
Only the address is resolved; the slot's value is never read.

Examples

nu.run(nu.AttrRef("x").exists())[0]

False

nu.run(nu.Let("x", 1, nu.AttrRef("x").exists()))[0]

True

Delete

Removes the slot its Ref names from the fabric, through that Ref.

Delete(ref)

Path nu.context.Delete. Kind Command, sort scalar_command, cardinality void. Arity 1 (1 required).

Same shape as SetCmd: the Ref is the declared mutation slot and the erase is delegated to it, so this drives any fabric Ref rather than only AttrRef.

Arguments

Name	Type	Default	Meaning
`ref`			the Ref naming the slot to remove. This is the mutation slot.

Yields

Nothing (VOID). The erase is the point.

Notes

Removing a slot that is not bound is a no-op, not an error.
Deleting is not the same as holding EMPTY: after a delete .exists() yields False, whereas a slot Let bound to EMPTY reads EMPTY and still exists.

Example

bind = nu.SetCmd(nu.AttrRef("a"), 1)
nu.run(nu.Sequential(bind, nu.Delete(nu.AttrRef("a"))))[1].attrs

Attributes()

Let

Binds a name to a value in ctx.attrs for the body's duration.

Let(name, value, body=None)

Path nu.context.Let. Kind Bracket, sort bracket, cardinality transparent. Arity 3 (2 required).

Evaluates value once, pushes it into ctx.attrs under name, runs body, then restores the prior slot on the way out - on a clean exit and on an exception alike. The body reads the binding back through AttrRef(name) (or any typed variant), so the one value can be dereferenced any number of times without recomputing it.

Arguments

Name	Type	Default	Meaning
`name`	`object`		evaluated at run time and required to be a `str`. A Python `str` is wrapped in a `Literal` at construction, so the common case is written with a plain name.
`value`	`object`		evaluated once, before the body runs.
`body`	`Nu \| None`	`None`	runs with the binding in place. Required despite the default.

Yields

Whatever body yields, in the body's own cardinality. Transparent like any Span: a Command body makes Let a writer, a stream body makes it a stream.

Notes

Scoped where SetCmd is open-ended. SetCmd leaves a slot on the context the run returns; a Let binding never leaks past its body. Reach for SetCmd when the write is the point, Let when the binding is a local.
Nesting shadows: an inner Let on the same name hides the outer one, and the outer value comes back when the inner body ends.
Unlike SetCmd, an EMPTY or INVALID value is still bound, so the slot exists and reads back as that sentinel.
Over a stream body the binding spans the whole drain, and is popped when the stream is exhausted.
Children are ordered [body, value, name]; the body sits in slot 0 to satisfy the Span transparency law.

Examples

nu.run(nu.Let("n", 7, nu.Add(nu.AttrRef("n"), 1)))[0]

nu.run(nu.Let("n", 2, nu.Let("n", 5, nu.Mul(nu.AttrRef("n"), 10))))[0]

nu.run(nu.Let("n", 7, nu.AttrRef("n")))[1].attrs

Attributes()

SetCmd

Writes a value into the slot its Ref names, through that Ref.

SetCmd(ref, value)

Path nu.context.SetCmd. Kind Command, sort scalar_command, cardinality void. Arity 2 (2 required).

The Command never touches ctx.attrs itself. It hands the Ref its own node id, the Ref resolves its address and performs the write, which is what lets SetCmd drive any fabric Ref and not just AttrRef.

Arguments

Name	Type	Default	Meaning
`ref`			the Ref naming the slot to write. This is the mutation slot, so effect synthesis binds it WRITE; every other slot is a read.
`value`			evaluated once, and its result is what lands in the slot.

Yields

Nothing (VOID). The write is the point.

Notes

An EMPTY or INVALID value writes nothing at all, so a slot that was already bound keeps whatever it held.
The write is open-ended: it lives on the context the run returns. Let is the scoped dual, binding only for a body's duration.

Examples

nu.run(nu.SetCmd(nu.AttrRef("total"), 10))[1].attrs

Attributes(total=10)

first = nu.SetCmd(nu.AttrRef("a"), 1)
nu.run(nu.Sequential(first, nu.SetCmd(nu.AttrRef("a"), nu.AttrRef("no"))))[1].attrs

Attributes(a=1)

attrs.interactions

AttrExists

Delete

Let

SetCmd

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