User:Zzo38/Proposal for more-notation

This document is proposal for more-notation in Haskell.

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.

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.

Uses

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;
};

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);