How to convert the type signature of a variable to a String?

[Marcin 'Qrczak' Kowalczyk]

Sun, 15 Apr 2001 14:20:01 +1000, Andrew J Bromage <andrew@bromage.org> pisze:

> We know that the most general type of `id' is "a->a".
> We assume there's a theoretical function:
> 
> 	get_the_function_type_of :: a -> String
> 
> where a can be a function type.  Now consider:
> 
> 	f :: (String -> String) -> String
> 	f g = g (get_the_function_type_of g)
> 
> The question is: Should `f id' return "a->a" or "String->String"?

Inside f the variable g has type String->String. It doesn't matter
that the expression used to build this argument could be used on
another type too: the level of concreteness of the type is not a
part of the object having that type, but a property of the variable
through which we access it.

It would return "String->String" (assuming that
get_the_function_type_of was in some sense possible). Any other
overloaded function would be also used as for String->String instead of
reporting an ambiguity error, where using it for 'id' directly could
report an ambiguity error if the instance is not defined uniformly
for all types of function arguments and results but depends on a
particular choice of them.

In other words all instances of an overloaded function must agree on
the level to which they examine the type. If one of them doesn't look
at a fragment of the type (e.g. function argument), others can't too;
if one of them depends on them, the type must be determined in order
to make a usage unambiguous.

ghc has -fallow-overlapping-instances which relaxes this requirement.
It allows fallbacks: when a type doesn't fit to any instance, try
to use a more general one. It still doesn't make calls which don't
determine that type unambiguous, and doesn't allow to recognize the
fact that a variable is polymorphic. It has some strange properties,
e.g. importing a module (which defines some instances) can change a
valid program into another valid program with a different meaning.

-- 
 __("<  Marcin Kowalczyk * qrczak@knm.org.pl http://qrczak.ids.net.pl/
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QRCZAK