Dear all, Conor wrote:
You forgot to define the term behaviour also. Pattern synonyms may be used to construct as well as to match values.
Conor and I discussed this over lunch. Specifically, we talked about whether the right hand side of a pattern synonym would be any Haskell pattern (including "_", "~", possibly "!"), or restricted to the intersection between the patterns and terms, as Conor propose that pattern synonyms also be used for construction. By adopting some simple conventions, like replacing "_" by "undefined" when a synonym is used as a term for construction, it is clear that one can be a bit more liberal than a strict intersection between the pattern and current expression syntax. Incidentally, this would be consistent with the way record patterns and record construction currently works. E.g. data Foo = MkFoo {l1 :: T1, l2 :: T2} A pattern "MkFoo {}" expands to "MkFoo _ _", a term "MkFoo {}" expands to "MkFoo undefined undefined". Moreover, didn't someone (John Mecham?) propose that "_" be a valid term anyway, standing for "undefined" (with an explicit requirement of keeping track of the source code position)? Maybe "~" (and "!") wouldn't cause much trouble either. I like the idea, but it would be nice if the RHS of a pattern synonym definition really coudl be any Haskell pattern, without any additional restriction (except that it has to be acyclic). All the best, /Henrik -- Henrik Nilsson School of Computer Science and Information Technology The University of Nottingham nhn@cs.nott.ac.uk This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.