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## Associating synonyms with types
This section is based upon [\#10653](https://gitlab.haskell.org//ghc/ghc/issues/10653).
Pattern synonyms allow for constructors to be defined, exported and imported separately from the types which they build.
However, it is sometimes convenient to associate synonyms with types so that we can more closely model ordinary data constructors.
If we want to refactor to change the internal representation of this maybe-like type to use Maybe.
```wiki
-- Main.hs
module Main where
import Internal ( A(..))
-- Internal.hs
module Internal where
data A = MkA Int | NoA
```
If we modify `Internal.hs` as follows
```wiki
{-# LANGUAGE PatternSynonyms #-}
module Internal where
newtype A = NewA (Just Int)
pattern MkA n = A (Just n)
pattern NoA = A Nothing
```
Then local definitions to `Internal` which used `A` would work as before but modules importing `Internal` and `A`
will no longer work as importing `A(..)` will import the type `A` and the constructor `NewA`. We can explicitly import the
new patterns but the usage of pattern synonyms should be transparent to the end user. What's needed is to be able to
associate the new synonyms with a type such that client code is oblivious to this implementation.
### Proposal
Richard proposes that synonyms are associated at the export of a datatype. Our running example would then look as follows:
```wiki
{-# LANGUAGE PatternSynonyms #-}
module Internal(A(MkA, NoA)) where
newtype A = NewA (Just Int)
pattern MkA n = A (Just n)
pattern NoA = A Nothing
```
### Specification
This proposal only changes module imports and exports.
#### Definition
We say that "a pattern synonym `P` is associated with a type `T` relative to module `M`" if and only if "`M` exports `T` whilst associating `P`".
#### Exports
For any modules `M``N`, we say that "`M` exports `T` whilst associating `P`" just when
- The export has the form `T(c1, ..., cn, P)` where c1 to cn (n \>= 0) are a mixture of other field names, constructors, pattern synonyms and the special token `..`. The special token `..`, which indicates either
1. all constructors and field names from `T`'s declaration, if `T` is declared in this module; or
1. all symbols imported with `T`, which might perhaps include patterns associated with `T` in some other module.
In case (2), `..` might in fact be a union of sets if `T` is imported from multiple modules with different sets of associated definitions.
#### Imports
For any modules `M``N`, if we import `N` from `M`,
- The abbreviated form `T(..)` brings into scope all the constructors, methods or field names exported by `N` as well any patterns associated with `T` relative to `N`.
- The explicit form `T(c1,...,cn)` can name any constructors, methods or field names exported by `N` as well as any patterns associated with `T` relative to `N`.
#### Typing
WIP
#### Clarification
- Associated patterns are typechecked to ensure that their type matches the type they are associated with.
- Hence, all synonyms must be initially explicitly associated but a module which imports an associated synonym is oblivious to whether they import a synonym or a constructor.
- According to this proposal, only pattern synonyms may be associated with a datatype. But it would be trivial to expand this proposal to allow arbitrary associations.
#### Examples
```
moduleN(T(..,P))wheredataT=MkTIntpatternP=MkT5-- M.hsmoduleMwhereimportN(T(..))
```
`P` is associated with `T` relative to `N`. M imports `T`, `MkT` and `P`.
```
moduleN(T(..))wheredataT=MkTIntpatternP=MkT5-- M.hsmoduleMwhereimportN(T(..))
```
`P` is unassociated. `M` imports `T` and `MkT`.
```
moduleN(T(P))wheredataT=MkTIntpatternP=MkT5-- M.hsmoduleMwhereimportN(T(..))
```
`P` is associated with `T` relative to `N`. M imports `T`, and `P`.
```
moduleN(T(P))wheredataT=MkTIntpatternP=MkT5-- M.hsmoduleM(T(..))whereimportN(T(..))-- O.hsmoduleOwhereimportM(T(..))
```
`P` is associated with `T` relative to `N`.
As `M` imports `N` and imports `T`, `P` is associated with `T` relative to `M`. Thus `M` exports `T` and `P`.
Therefore when `O` imports `T(..)` from `M`, it imports `T` and `P`.
```
moduleN(T(..))wheredataT=MkTInt-- M.hsmoduleM(T(P))whereimportN(T(..))patternP=MkT5-- O.hsmoduleOwhereimportM(T(..))
```
This example highlights being able to freely reassociate synonyms.
`M` imports `T` and `MkT` from `N` but then as `M` associates `P` with `T`, when `O` imports `M`, `T` and `P` are brought into scope.
### Unnatural Association
There is some discussion about what should happen with synonyms which target types defined in the prelude.
It is very uncommon to explicitly import datatypes defined in the prelude, thus this kind of association is very rare in practice
but should be allowed.
### Module Chasing
Simon is a bit worried that once we allow this association then there is no limit to the number of constructors which
can be associated with a type. With normal datatypes, `T(..)` means some subset of the constructors defined where `T` is
defined. With this proposal `T(..)` has no maximal meaning, I don't see any problem with this behaviour as the meaning can still
be determined by the renamer.
### The Privilege Objection
Simon also wonders why it is possible to privilege pattern synonyms in this way and not normal functions for example.
I don't think this is particularly puzzling as at their most general pattern synonyms allow for the definition of unassociated
data constructors. Being able to later associate them with types only brings their behaviour closer to ordinary data constructors.
### Polymorphic Synonyms
The following is a valid pattern synonym declaration which doesn't have a definite constructor.
```wiki
{-# LANGUAGE PatternSynonyms, ViewPatterns #-}
module Foo where
class C f where
build :: a -> f a
destruct :: f a -> a
pattern P :: () => C f => a -> f a
pattern P x <- (destruct -> x)
where
P x = build x
```
I propose that we allow such synonyms to be associated with a type `T` as long as it typechecks. I don't expect this to be much used in practice.
### Associatation at definition
Simon proposed that synonyms are associated at the definition of a datatype. Our running example would look as follows:
```wiki
{-# LANGUAGE PatternSynonyms #-}
module Internal(A(MkA, NoA)) where
newtype A = NewA (Just Int)
with (MkA, NoA)
pattern MkA n = A (Just n)
pattern NoA = A Nothing
```
The proposal refers to Richard's suggestion rather than Simon's refinement for the following reasons.
Consider two packages `old-rep` and `new-rep` which have different representations of the same structure.
The library author wants to smooth the transition for his users by providing a compatibility package `compat-rep`
so that code using the old representation in `old-rep` can work seamlessly with `new-rep`.
The problem is to define `compat-rep` such that by changing the dependencies of our package, our code to continues to work
but without depending on `old-rep`. More generally, an author may want to write a `*-compat` package for two packages which they do
not control. Having to define these synonyms at the definition site is too restrictive for this case .
See [PatternSynonyms/AssociatingSynonyms](pattern-synonyms/associating-synonyms)
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