ADR: Total conversion functions conventions

Author: carbolymer

docs/ADR-014-Total-conversion-functions-conventions.md

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+# Status
+
+- [ ] Proposed 2024-05-21
+
+# Context
+
+In `cardano-api` we have multiple functions performing conversions between one value of the type to the other, for example:
+
+```haskell
+fromShelleyDeltaLovelace :: L.DeltaCoin -> Lovelace -- 'from' at the beginning
+lovelaceToQuantity :: Lovelace -> Quantity -- 'to' in the middle
+toAlonzoScriptLanguage :: AnyPlutusScriptVersion -> Plutus.Language -- 'to' at the beginning
+convReferenceInputs :: TxInsReference build era -> Set Ledger.TxIn -- 'conv' at the beginning
+```
+
+There are multiple naming conventions for the conversion functions which makes them hard to locate.
+Some conversion functions with lengthy names, are not very convenient to use.
+
+# Decision
+
+## Type classes
+
+For total functions, which are simply converting a value from one type to another, we can use type classes [`Inject` (from `cardano-ledger`)](https://cardano-ledger.cardano.intersectmbo.org/cardano-ledger-core/Cardano-Ledger-BaseTypes.html#t:Inject) & [`Convert`](https://cardano-api.cardano.intersectmbo.org/cardano-api/Cardano-Api-Internal-Eras.html#t:Convert):
+```haskell
+class Inject t s where
+  inject :: t -> s
+
+class Convert (f :: a -> Type) (g :: a -> Type) where
+  convert :: forall a. f a -> g a
+```
+
+The use of those conversion functions should be limited to **internal use only**.
+The library should still export conversion functions with explicit type names for better readability.
+An exception to this would be a set of types which are convertible one to the other, like `Eon`s.
+Writing $N \times N$ conversion functions for $N$ types would be cumbersome and using `inject`/`convert` instead is justified here.
+
+Inject instances should be placed near one of the types definition, to make them more discoverable and avoid orphaned instances.
+
+>[!NOTE]
+>The difference between `Inject` and `Convert` class is that `Convert` is better typed for types with `Type -> Type` kind.
+>In other words, when writing `instance Inject (Foo a) (Bar a)` the GHC's typechecker needs some help to understand the code using `inject`:
+>```haskell
+>let x = inject @_ @(Bar Bool) $ Foo True
+>```
+>That is not needed for `convert`.
+
+### Injection law
+
+The `Inject` and `Convert` classes are meant to be used for trivial conversions only and not for more complex types like polymorphic collections (e.g. `Set a`).
+The `inject` and `convert` implementations should both be injective:
+```math
+\forall_{x,x' \in X} \ \ inject(x) = inject(x') \implies x = x'
+```
+
+This effectively means that any hashing functions or field accessors for constructors losing information (e.g. `foo (Foo _ a) = a`) should not be implemented as `Inject`/`Convert` instances.
+
+## Explicit conversion functions
+
+For explicit conversion functions, the following naming convention should follow:
+
+```haskell
+fooToBar :: Foo -> Bar
+```
+
+### Qualified imports
+If the module exporting conversion functions is meant to be imported qualified, and provides functions for operating on a single data type, a shorter name with `from` prefix is allowed:
+
+```haskell
+module Data.Foo where
+
+fromBar :: Bar -> Foo
+```
+
+where the usage would look like:
+```haskell
+import Data.Foo qualified as Foo
+
+Foo.fromBar bar
+```
+
+# Consequences
+
+## Advantages
+- An uniform API for total conversions
+- A list of `Inject` instances lists all available conversions for the type
+- Less maintenance burden with regards to the naming conventions of the conversion functions
+
+## Disadvantages
+- It may be a bit surprising how to discover available conversions, because one would have to browse the type's `Inject` instances to find the conversion functions they are looking for - instead of looking for exported functions.
+
+
+[modeline]: # ( vim: set spell spelllang=en: )