Propose ADR 0001: Remove isinstance checks when setting parameters

docs/developer/adr/0001-remove-isinstance-checks-when-setting-parameters.md

@@ -0,0 +1,68 @@
+# ADR 0001: Remove isinstance checks when setting parameters
+
+- Status: proposed
+- Deciders: Jan-Lukas, Neil, Simon
+- Date: 2024-04-15
+
+## Context
+
+Sciline builds a data dependency graph based on type hints of callables.
+Dependencies can be fulfilled by setting values (instances of classes) as so called *parameters*.
+In an attempt to extend the correctness guarantees of the dependency graph, Sciline's `__setitem__` checks if the value is instance of the key (a type) when setting a parameter.
+
+This has led to a number of problems.
+For example, supporting different file handles types is too difficult [#140](https://github.com/scipp/sciline/issues/140),
+parameter type handling is too inflexible in general [#144](https://github.com/scipp/sciline/issues/144),
+and the mechanism is broken with Python 3.12 type aliases [#145](https://github.com/scipp/sciline/issues/145).
+In short, the mechanism gets in the way of the user, since it causes false positives.
+
+Considering the bigger picture, we can think of this mechanism as a poor man's form of *validation*.
+Validation of input parameters is very important when running workflows, but it should be done in a more explicit way.
+Validating the type is only a fraction of what we want to do when validating parameters.
+Therefore, we should remove this mechanism and replace it with a more general validation mechanism.
+The more general validation mechanism can be considered out of scope for Sciline, and should be implemented in the user code or using other common libraries such as `pydantic`.
+
+Finally, we can think of this mechanism as a form of runtime type checking.
+We should ask ourselves if this is the intended scope of Sciline.
+If it is, shouldn't we also check that each provider actually returns the correct type?
+
+The main problem with not checking value types when setting parameters is that it is not possible to catch such errors with `mypy`, in contrast to return values of providers, which `mypy` *can* check.
+
+Consider the following example of setting $Q$ bins for a workflow, given by a `scipp.Variable`, which would then be passed to `scipp.hist` to create a histogram:
+
+```python
+pipeline[QBins] = sc.linspace(...)
+pipeline[QBins] = 1000  # error in current implementation
+pipeline[QBins] = sc.linspace(..., unit='m')  # no error, but wrong unit
+```
+
+Checking the type catches the first error, but not the second.
+Paradoxically, setting an integer would often be a valid operation in the example, since `scipp.hist` can handle this case, whereas the wrong unit would not be valid.
+This may indicate that defining `QBins` as an alias of `scipp.Variable` is actually an anti-pattern.
+Instead, imagine we have defined a specific `class QBins`, which performs validation in its constructor, and defines `__call__` so it can be used as a provider:
+
+```python
+pipeline.insert(QBins(sc.linspace(...)))
+pipeline.insert(QBins(1000))  # ok
+pipeline.insert(QBins(sc.linspace(..., unit='m')))  # error constructing QBins
+```
+
+This example illustrates that a clearer and more specific expression of intent can avoid the need for relying on checking the type of the value when setting a parameter.
+
+## Decision
+
+- Remove the mechanism that checks if a value is an instance of the key when setting it as a parameter.
+- Encourage users to validate inputs in providers, which can also be tested in unit tests without setting up the full workflow.
+- Encourage users to use a more general parameter validation mechanism using other libraries.
+- Consider adding a mechanism to inject a callable to use for parameter validation as a argument when creating a `Pipeline`.
+
+## Consequences
+
+### Positive
+
+- The mechanism will no longer get in the way of the user.
+- The code will be simplified slightly.
+
+### Negative
+
+- `sciline.Pipeline` will support duck-typing for parameters, in a way that cannot be checked with `mypy`.

docs/developer/architecture-decision-records.md

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+# Architecture Decision Records
+
+```{toctree}
+---
+maxdepth: 1
+glob: true
+---
+
+adr/*
+```

docs/developer/index.md

@@ -14,4 +14,5 @@ getting-started
 coding-conventions
 dependency-management
 architecture-and-design/index
+architecture-decision-records
 ```