parameterized patterns

[lutzhamel]

An idea on how to look at parameterized patterns. Currently Asteroid supports parameterized patterns via variables read from the environment,

Asteroid Version 2.0.1alpha
(c) University of Rhode Island
Type "asteroid -h" for help
Press CTRL-D to exit
ast> let p = pattern %integer.
ast> let pair = pattern (x:*p,y:*p).
ast> let q:*pair = (1,2). -- here pair is a pair of integer values
ast> q
(1,2)
ast> let p = pattern %real. -- now pair is a pair of real values
ast> let q:*pair = (1,2).
error: pattern match failed: conditional pattern match failed
ast> let q:*pair = (1.1,2.2).
ast> 

This is very clumsy and prone to programming errors...think dynamically scoped functions. A nice way would be to actually have have parameters on the pattern. Something like this perhaps,

ast> let p = pattern %integer.
ast> let pair = pattern with a match (x:*a,y:*a).
ast> let q:*pair(p) = (1,2). -- here pair is a pair of integer values
ast> q
(1,2)

or in more concise form,

ast> let pair = pattern with a match (x:*a,y:*a).
ast> let q:*pair(pattern %integer) = (1,2). -- here pair is a pair of integer values
ast> q
(1,2)
ast> let q:*pair(pattern %real) = (1.1,2.2). -- here pair is a pair of real values
ast> q
(1.1,2.2)

[lutzhamel]

We cannot use juxtaposition as a way to parameterize patterns, it makes the language too ambiguous due to the confusion with function application. We can adopt the generics notation here,

ast> let pair = pattern<a> (x:*a,y:*a).
ast> let q:*pair<pattern %integer> = (1,2). -- here pair is a pair of integer values
ast> q
(1,2)
ast> let q:*pair<pattern %real> = (1.1,2.2). -- here pair is a pair of real values
ast> q
(1.1,2.2)