# [GT4Py] Flexible Embedded Backends

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- Shaped by: Hannes
- Appetite (FTEs, weeks): 1 cycle, 1 dev + 1 for knowledge transfer
- Developers: <!-- Filled in at the betting table unless someone is specifically required here -->

## Problem

The toolchain steps where put into workflows in the previous cycle.
Still some steps, specifically the ones checking the syntactical/type correctness, are hard-coded in the `field_operator`/`program` decorators.

The desired state would be to allow full customization of the toolchain via workflow steps by specifying a backend (TODO: to be renamed to `workflow`, `toolchain`, `transforms`, `???`?).

This would allow pure embedded (without linting through parsing+lowering+type deduction) without hacks.

This project also serves as knowledge transfer for the workflowified toolchain.

## Appetite

1 cycle

## Solution

Fixing the problem is both a cleanup and a feature:
- remove special-casing around "embedded backends"
- enable full customization

The linting steps which are currently in `post_init`s in decorators will be removed. Instead these steps become part of a backend (This is already in the pipeline: [#1516](https://github.com/GridTools/gt4py/pull/1516), only the calls remain in the decorators). A fully `None` backend will not do any checks or transformations.
To keep early checking we add an embedded backend with a linting step. To keep current behavior this will become the new default backend.
To be able to switch the default we need a customization option for the default backend, to allow field_operators to have non-checked backend even without explicitly setting a backend on each of them.

Additional design questions, for follow-up cleanup:
- clear separation of workflows that run at different times:
- at load of the function: linting
- at compile time (to enable AOT compilation, should not use runtime \*arg \*\*kwargs, only in case of jit)
- at run time (should only call?)

## Rabbit holes
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Having the no-checks embedded execution represented internally by a `None`-valued backend might not be achievable without special casing in the decorator classes.
Mitigate by time-boxing any exploration into this and else choose to convert a value of `None` passed to the decorator functions to another representation of the no-checks embedded execution (some kind of backend instance).

## No-gos
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## Progress
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- [x] Task 1 ([PR#xxxx](https://github.com/GridTools/gt4py/pulls))
- [x] Subtask A
- [x] Subtask X
- [ ] Task 2
- [x] Subtask H
- [ ] Subtask J
- [ ] Discovered Task 3
- [ ] Subtask L
- [ ] Subtask S
- [ ] Task 4