|Version 2 (modified by simonmar@…, 7 years ago) (diff)|
Lift the monomorphism restriction
We could simply remove the M-R from the language, but suggest that implementations issue a warning when a loss of sharing might occur due to overloading of a variable or pattern binding.
The warning would not be mandatory - the report doesn't mandate implementation behaviour of this kind, rather it would be a recommendation. To make the warning mandatory would require talking about operational semantics, which the report also doesn't do.
- Simple, removes a wart from the language
- "As much polymorphism as possible" is in the spirit of Haskell - it is strange to limit polymorphism for performance reasons (indeed, this is arguably a poor compromise)
- It turns out to be quite hard to demonstrate a performance problem due to the M-R, at least with GHC, because its optimiser often recovers the sharing.
- Even if you do get loss of sharing, profiling will quickly pinpoint it
- nhc98 has never implemented the M-R, and users haven't found any significant problems as a result
- Hugs has a different (non-Haskell98) implementation of the M-R
- Haskell doesn't specify an evaluation strategy so introducing the concept of 'sharing' is strange indeed.
- Might be hard to give an accurate warning; just warning about overloaded variable bindings isn't good enough, because they don't all result in loss of sharing.
- The warning might be confusing to new users (but if we could make it accurate, it wouldn't happen much)
- For cases where you want the polymorphic type and don't want to write a type signature, then having the compiler emit a warning by default is undesirable.