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# Empty data declarations
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**Ticket:**[\#25](https://gitlab.haskell.org//haskell/prime/issues/25)
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## Brief Explanation
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# Proposal: [EmptyDataDecls](empty-data-decls)
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<table><tr><th> Ticket </th>
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<th>[\#25](https://gitlab.haskell.org//haskell/prime/issues/25)</th></tr>
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<tr><th> Dependencies </th>
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<th> names of other proposals on which this one depends
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</th></tr>
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<tr><th> Related </th>
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<th>
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<table><tr><th>[\#85](https://gitlab.haskell.org//haskell/prime/issues/85)</th>
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<td>more liberal kind inference</td></tr></table>
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<table><tr><th>[\#84](https://gitlab.haskell.org//haskell/prime/issues/84)</th>
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<td>Kind Annotations</td></tr></table>
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<table><tr><th>[\#78](https://gitlab.haskell.org//haskell/prime/issues/78)</th>
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<td>Add infix type constructors</td></tr></table>
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Compiler support GHC \[ full (flagname) \| partial (flagname) (comments) \| none \]
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nhc98 \[ full (flagname) \| partial (flagname) (comments) \| none \]
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Hugs \[ full (flagname) \| partial (flagname) (comments) \| none \]
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UHC \[ full (flagname) \| partial (flagname) (comments) \| none \]
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JHC \[ full (flagname) \| partial (flagname) (comments) \| none \]
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LHC \[ full (flagname) \| partial (flagname) (comments) \| none \]
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Summary
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Allow data declarations with no constructors.
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Description
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The proposal is to allow empty `data` declarations, i.e. data types without any
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constructors. Syntactically, it is basically just a matter of making the
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"`=`*constrs*" part optional in the context free syntax. Semantically,
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the result is a type (once the type constructor has been fully applied)
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whose only element is bottom. Examples (assuming
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[Infix Type Constructors](infix-type-constructors) is adopted):
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```wiki
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data S
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data T a
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data a :*: b
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data a :\*: b
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data (a :**: b) c
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```
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data (a :\*\*: b) c
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[Kind inference](kind-inference) will of course be carried out for types constructors
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introduced by empty declarations just as for any other type constructors according to
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whatever rules are adopted. Unless there are further constraints, the kinds of the
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constructors above would be
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```wiki
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S :: *
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T :: * -> *
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(:*:) :: * -> * -> *
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(:**:) :: * -> * -> * -> *
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```
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S :: \*
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T :: \* -\> \*
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(:\*:) :: \* -\> \* -\> \*
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(:\*\*:) :: \* -\> \* -\> \* -\> \*
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If [kind annotations](kind-annotations) are adopted, they should obviously also apply
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to empty declarations. They would possibly be a little more important for empty declarations,
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| ... | ... | @@ -45,11 +58,9 @@ is adopted. [Polymorphic kinds](kind-inference) would make this point moot, thou |
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annotations then never would be needed, unless it is decided that kind annotations still would
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be good documentation.
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Note that contexts of course also would be allowed, but, as there are no data constructors, their
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only impact would be on the inferred kind.
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The only real issue is whether or not to allow the optional `deriving` clause after an empty declaration,
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and, if not, on what stage to rule them out. Clearly, as there are no data constructors over which to
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define functions, derived instances would (for consistency) have to be everywhere undefined. GHC seems
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| ... | ... | @@ -57,17 +68,13 @@ to syntactically allow a `deriving` clause after an empty data declaration, but |
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contextual error since no interesting instances can be defined. Presumably the reasoning was that this
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gives a more regular syntax and better error messages than ruling out deriving for empty declarations
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syntactically. But the point is that there is a choice.
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A simple and natural generalisation of data declarations, seemingly without any hidden complications.
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References[ http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html\#nullary-types](http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#nullary-types) GHC documentation.
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Report Delta
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The changes to the report necessary to implement the proposal. Required for a proposal to move to the "accepted" state.
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Download in other formats:[Plain Text](/wiki/EmptyDataDecls?version=6&format=txt)[](http://trac.edgewall.org/)Powered by [Trac 1.2.2](/about)
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## References
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- [ http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html\#nullary-types](http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#nullary-types) GHC documentation.
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-
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<table><tr><th>[\#85](https://gitlab.haskell.org//haskell/prime/issues/85)</th>
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<td>more liberal kind inference</td></tr></table>
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[\#84](https://gitlab.haskell.org//haskell/prime/issues/84)Kind Annotations[\#78](https://gitlab.haskell.org//haskell/prime/issues/78)Add infix type constructorsProsA simple and natural generalisation of data declarations, seemingly without any hidden complications.
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ConsDownload in other formats:[Plain Text](/wiki/EmptyDataDecls?version=5&format=txt)[](http://trac.edgewall.org/)Powered by [Trac 1.2.2](/about)
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By [Edgewall Software](http://www.edgewall.org/).Visit the Trac open source project at
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[http://trac.edgewall.com/](http://trac.edgewall.com/)
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By [Edgewall Software](http://www.edgewall.org/).Visit the Trac open source project at
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[http://trac.edgewall.com/](http://trac.edgewall.com/) |
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</th></tr></table> |
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\ No newline at end of file |
