-
Notifications
You must be signed in to change notification settings - Fork 7
ADR: Total conversion functions' conventions #74
New issue
Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.
By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.
Already on GitHub? Sign in to your account
Merged
carbolymer
merged 1 commit into
main
from
mgalazyn/adr-use-inject-for-conversion-functions
Jun 9, 2025
Merged
Changes from all commits
Commits
File filter
Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
There are no files selected for viewing
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Original file line number | Diff line number | Diff line change |
---|---|---|
@@ -0,0 +1,118 @@ | ||
# Status | ||
|
||
- [ ] Proposed 2024-05-21 | ||
|
||
# Context | ||
|
||
In `cardano-api` we have multiple functions for performing conversions on values from one type to another, 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. | ||
carbolymer marked this conversation as resolved.
Show resolved
Hide resolved
|
||
An exception to this would be a set of types which are all convertible to each other, like `Eon`s. | ||
Writing $N \times N$ conversion functions for $N$ types would be cumbersome, so using `inject`/`convert` instead is justified here. | ||
|
||
Inject instances should be placed near the definition of one of the types, 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. | ||
carbolymer marked this conversation as resolved.
Show resolved
Hide resolved
|
||
>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. `[a] -> Set a` which loses ordering). | ||
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 | ||
``` | ||
|
||
>[!IMPORTANT] | ||
>Conversion functions should be placed near the conversion target type definition if possible. | ||
|
||
### 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` or `to` prefix is allowed. | ||
|
||
#### `from`-prefixed functions | ||
|
||
For defining conversion functions, **using `from`-prefixed functions should be preferred**. | ||
The `from...` function should be placed nearby the `Foo` definition. | ||
|
||
```haskell | ||
module Data.Foo where | ||
|
||
import Data.Bar (Bar) | ||
|
||
data Foo = Foo | ||
|
||
fromBar :: Bar -> Foo | ||
``` | ||
|
||
where the usage would look like: | ||
```haskell | ||
import Data.Foo qualified as Foo | ||
|
||
Foo.fromBar bar | ||
carbolymer marked this conversation as resolved.
Show resolved
Hide resolved
|
||
``` | ||
|
||
#### `to`-prefixed functions | ||
|
||
When it's not possible to define `from`-prefixed functions in the location of the target type, it's permitted to use `to`-prefixed function. | ||
|
||
```haskell | ||
module Data.Foo where | ||
|
||
import Data.Baz (Baz) | ||
data Foo = Foo | ||
|
||
toBaz :: Foo -> Baz | ||
``` | ||
|
||
# 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 less obvious 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: ) |
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Uh oh!
There was an error while loading. Please reload this page.