0000109: Polymorphic Variant generates comiler exception

Notes
(0000387) administrator (administrator) 2000-05-12 04:52	> Full_Name: David Clarke > Version: 3.00 > OS: SunOS > Submission from: cache2.uwn.unsw.edu.au (129.94.6.29) > > > The following code (which is invalid): > > let f x = match x with `spam 3 -> 29 \| `spam "spat" -> 23;; > > causes the compiler to halt and generate the following error message: > > Uncaught exception: File "typing/parmatch.ml", line 0, characters 16649-16661: > Assertion failed Well, the current definition is a bit ambiguous about what is valid or not. In particular, the definition let f1 = function `spam 3 -> 29 let f2 = function `spam "spat" -> 23 let f x = f1 x + f2 x val f1 : [< `spam of int] -> int = <fun> val f2 : [< `spam of string] -> int = <fun> val f : [< `spam of string & int] -> int = <fun> is explicitly described as valid, while it is of course meaningless. The bug you report could be solved in two ways: * either decide that this is valid, and just produces an unusable function. This requires compiling it to some arbitrary code, and would make some parts of the compiler rather dirty. * or explicitly say that while variant types with conjunctive types are allowed in expressions, they are not allowed in patterns. This gives a more intuitive behaviour, with useful error messages. I think that the second solution is better, and I have implemented it in the CVS version, by using a stricter form of unification on pattern types. However this gives raise to some philosophical questions about what are the theoretical properties expected when typing patterns, and how this interacts with the typing of expressions. This change is subject to theory and implementation improvements. Jacques Garrigue

(0000388) administrator (administrator) 2000-05-24 03:38	Fixed on 2000-05-12 by Jacques (changed typing of variants in patterns, to allow only one type for matched tags).

Issue History
Date Modified	Username	Field	Change
2005-11-18 10:13	administrator	New Issue

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