User:Zzo38/Proposal for more-notation
From HaskellWiki
This document is proposal for more-notation in Haskell.
Contents |
1 Syntax
Anywhere that a more-notation is allowed (see Uses), you can use the syntax:
- more [(name_of_enumeration)] more_name {parameters}
- name_of_enumeration: A name of any type which is an enumeration. It can be another one using more-notation, but it must be able to fully determine it before this more-notation can be fully determined.
- more_name: A capitalized name. It uses the same namespace as constructors, so you are not allowed to have a constructor of the same name (although it is OK to have types of the that name).
- parameters: Optional lowercased names. These names are in local scope, and may already exist in this scope, although they do not have to.
A more-declaration is a module-level declaration of this syntax:
- [ numeric_literal | (enumeration_constructor) ] more_name {parameters} = contents { | contents } ;
- numeric_literal: It should be a natural number. Omitted is the same as zero. This number is called the "order" of the declaration.
- enumeration_constructor: A constructor of the enumeration that was specified in the more-notation. If an enumeration is specified, the enumeration constructor is required here. This number is called the "order" of the declaration.
- parameters: Lowercased names which will be scoped locally to this declaration. The number of parameters must match the number of parameters specified in the more-notation. An unused parameter can be replaced by an underscore.
- contents: The contents depends on where the more-notation is.
2 Semantics
In the place where the more-notation is, all contents of more-declarations of that name will be reordered and corrected before being placed in place of the more-notation.
Automatic reordering is overridden by the enumeration constructors or numeric literals in front of more-declarations.
3 Uses
3.1 Do-blocks
3.2 Case alternatives
An alternative for a case block may be replaced by a more-notation. Semantics of more-declarations becomes as follows:
- Cases within an order are reordered so that the most specific ones come first.
Example:
X = (_, False, True) -> 1; X = (True, True, True) -> 2; X = _ -> 3; X = (False, True, True) -> 4; y x = case x of { more X; };
becomes:
y x = case x of { (True, True, True) -> 2; (False, True, True) -> 4; (_, False, True) -> 1; _ -> 3; };
3.3 Datatype declarations
A constructor definition in a data declaration is allowed to be replaced by a more-notation. Semantics of more-declarations becomes as follows:
- Duplicate constructors are removed.
- Duplicate constructors that do not match are errors.
- Multiple constructors in a single more-declaration are guaranteed to keep the order given.
Example:
data T = Zero | more T deriving (Eq); T = Two | Three | Four; T = One | Two;
becomes
data T = Zero | One | Two | Three | Four deriving (Eq);