Rob Arthan wrote:
I have a small issue with the Poly/ML pretty-printer. If you run:
datatype FOO = D of (int * int); infix D; 1 D 2;
Poly/ML prints: val it = D (1, 2) : FOO
Poly/ML will parse D(1, 2) but only because of a non-standard
extension that lets you omit the keyword op that the Standard ML
definition requires before infix operator names used with ordinary
function application syntax.
Poly/ML does a reasonable job of printing expressions in a way that
enables them to be parsed but there's no guarantee that the output can
be parsed without first being manually edited. It's easy enough to
construct examples where a value can be printed but the constructors are
not actually in scope.
Prior to version 5.3 Poly/ML did take account of the infix environment
when printing constructors but that changed in the 5.3 release. From
5.3 the compiler produces a default print function when a datatype is
declared and that always uses the prefix form. I did consider having
this use an infix form if a constructor had infix status when the
datatype was declared but decided that that was only going to be
confusing. It wouldn't deal with your example anyway.
Before version 5.3 printing was done by interpreting the type
information when the value was actually printed so it was possible to
choose either prefix or infix form according to the fixity status at the
point of printing. For this to work, though, various hacks were
required particularly in the representation of types inside the compiler
so that it was possible to find the constructor environment, the set of
value constructors and their types for a datatype, from the type of a
value. That isn't something that is needed anywhere else and the hacks
had various undesirable side-effects such as on printing type
information in error messages. I decided to change this to have a
printing function associated with the datatype, and also an equality
function - removing the need for structure equality. That then
removed the need to be able to get from the type of a value to the
constructor environment.
Basically, if you need a value printed out in infix form you've always
got the option of installing your own pretty-printer for the type.
Regards,
David
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