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Changes between Version 4 and Version 5 of PatternSynonyms

Timestamp:: 08/15/11 14:57:07 (22 months ago)
Author:: augustss
Comment:: Update syntax, add associated patterns.

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PatternSynonyms

v4	v5
67	67	The pattern only synonyms can have any pattern on the right hand side, but may only be used in patterns.
68	68
69		`pattern` ''conid'' ''varid,,1,,'' ... ''varid,,n,,'' `~` ''pat''
	69	`pattern` ''conid'' ''varid,,1,,'' ... ''varid,,n,,'' `=` ''pat''
70	70
71		~~Note the use of `~` instead of `=` as the equality symbol. This serves as a syntactic cue that this is a pattern only synonym.~~
72	71	Again, each of the variables on the left hand side must be mentioned exactly once of the right hand side, but the right hand side can mention other variables as well. These variables will not be bound by using the pattern synonyms.
73	72
74	73	Examples:
75	74	{{{
76		pattern ThirdElem x ~ _:_:x:_
	75	pattern ThirdElem x = _:_:x:_
77	76	pattern LazySecond a b ~ (a, ~b)
78	77
…	…
94	93	Together with ViewPatternsAlternative we can now create patterns that look like regular patterns to match on existing (perhaps abstract) types in new ways.
95	94	{{{
96		pattern Plus1 n ~ n1 \| let n = n1-1, n >= 0
	95	pattern Plus1 n = n1 \| let n = n1-1, n >= 0
97	96
98	97	fac 0 = 0
…	…
107	106	This is the rational for the most complicated synonyms, the bidirectional ones. They provide two expansions, one for patterns and one for expressions.
108	107
109		`pattern` ''conid'' ''varid,,1,,'' ... ''varid,,n,,'' `~` ''pat'' `where` ''cfunlhs'' ''rhs''
	108	`pattern` ''conid'' ''varid,,1,,'' ... ''varid,,n,,'' `=` ''pat'' `where` ''cfunlhs'' ''rhs''
110	109
111	110	where ''cfunlhs'' is like ''funlhs'', except that the functions symbol is a ''conid'' instead of a ''varid''.
…	…
113	112	Example:
114	113	{{{
115		pattern Plus1 n ~ n1 \| let n = n1-1, n >= 0 where
	114	pattern Plus1 n = n1 \| let n = n1-1, n >= 0 where
116	115	Plus1 n = n + 1
117	116	}}}
…	…
123	122	}}}
124	123
	124	== Associated Patterns Synonyms ==
	125	Just like data types and type synonyms can be part of a class declaration, it would be possible to have pattern synonyms as well.
125	126
	127	Example:
	128	{{{
	129	class ListLike l where
	130	pattern Nil :: l a
	131	pattern Cons :: a -> l a -> a
	132	isNil :: l a -> Bool
	133	isNil Nil = True
	134	isNil (Cons _ _) = False
	135	append :: l a -> l a -> l a
	136
	137	instance ListLike [] where
	138	pattern Nil = []
	139	pattern Cons x xs = x:xs
	140	append = (++)
	141	}}}