[m-dev.] diff: changes to term_to_type & type_to_term

[Thomas Charles CONWAY]

Fergus Henderson, you write:
> Hi,
> 
> Tom, can you please review this one?
> 
> Incidentally, the code in this change shows the need for existential types.
> There were several places where without existential types, the only way
> to make the code do what I wanted it to do was to use unsafe_cast and
> friends.
> 
> -----------------------------------------------------------------------------
> 
> Various improvements to term_to_type, type_to_term, and io__write,
> particularly with regard to the handling of data of type uniq_array(T).
> 
> library/term.m:
> 	- Improve the error messages available from term_to_type: add
> 	  a new predicate `try_term_to_type' which is like term_to_type
> 	  but which returns much more detailed information rather than
> 	  failing.  (XXX not yet done: we should modify io__read to take
> 	  advantage of this.)
> 	- Extend term_to_type to handle uniq_array/1.
> 	- Extend type_to_term to handle uniq_array/1, univ/0, and type_info/0.
> 	- Rename term__term_to_type_2 as term__univ_to_type and export
> 	  it (it is needed by io.m).
> 
> library/io.m:
> 	- Fix a bug in the output of uniq_arrays: call term__univ_to_type
> 	  rather than term__term_to_type.
> 

This looks good.
Should io__read give back a term_to_type_result?

Thomas
-- 
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Thomas Conway               				      conway at cs.mu.oz.au
AD DEUM ET VINUM	  			      Every sword has two edges.