7.9. Implicit parameters

Implicit paramters are implemented as described in "Implicit parameters: dynamic scoping with static types", J Lewis, MB Shields, E Meijer, J Launchbury, 27th ACM Symposium on Principles of Programming Languages (POPL'00), Boston, Jan 2000.

There should be more documentation, but there isn't (yet). Yell if you need it.

• You can't have an implicit parameter in the context of a class or instance declaration. For example, both these declarations are illegal:

  class (?x::Int) => C a where ... instance (?x::a) => Foo [a] where ...

  Reason: exactly which implicit parameter you pick up depends on exactly where you invoke a function. But the ``invocation'' of instance declarations is done behind the scenes by the compiler, so it's hard to figure out exactly where it is done. Easiest thing is to outlaw the offending types.