Subject: Re: Export lists in modules
Type signatures currently do more than state the type of a variable.
They potentially (i) enable polymorphic recursion, and (ii) disable the
monomorphism restriction, thus losing sharing. It would be very strange
if a type signature *in an export list* were to have these effects--the
export list surely should not change the way a module is compiled and
type-checked. One good principle is that commenting out the export list
should not change the way any definition is compiled, only what is
exported. After all, commenting out the exports (so that all definitions
can be tested) is a common step when debugging a module.
I would conclude that polymorphic recursion should be enabled only if a
type signature appears *in the body of the module*, similarly the M-R
should be disabled only if an overloaded type signature appears in the
body of the module.
The same principle should apply to any other type signatures that
change the way a definition is type-checked, if they are included in
Haskell', such as rank-2 or rank-N type signatures.
John
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