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RE: Return type of procedures?
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Date: -- (:)
From: Manuel Fahndrich <maf@m...>
Subject: RE: Return type of procedures?
We are having a related problem with unit and void in a language we design
at MSR called Vault. The problem we ran into was with polymorphism. If you
have a polymorphic function in your language (such as the identity id : 'a
-> 'a), you could still end up in the situation you are trying to avoid,
namely by saying:

	id (p ()),

where p : unit -> void

In this example, one could check the instance type, but in general, one can
hide these instances inside other polymorphic functions and procedures and
that approach is impractical.


-----Original Message-----
From: John Max Skaller []
Sent: Monday, August 21, 2000 10:17 AM
Subject: Return type of procedures?

I am designing a programming language (the compiler is written
in ocaml) which is a procedural language with 'purely functional'
expressions (using eager evaluation). Function closures can
access their context, which procedural statements my change between
building the closure and evaulating it. Procedural closures may
mutate their environment.

Procedures and functions are defined with distinct keywords
'procedure' and 'function'. I have given a procedure the 
functional type

	'a -> unit (where 'a may sensibly be 'unit')

whereas functions have signature

	'a -> 'b

where either 'a or 'b may be unit. Note that if 'a is unit, the
function is not necessarily constant, since it may depend on
the environment. OTOH, a function with 'b as unit is farily

However, as it stands, the type system fails to achieve
separation of functions and procedures:

	procedure p: unit -> unit
	function f: unit -> unit
	f (p ())  // woops! 

Here is an expression (functional code) which has taken the result
of calling a procedure (unit) as an argument.

Proposal: just as unit is the trivial product (tuple, terminal object),
we say 'void' is the trivial sum (initial object), and make procedures
have the signature

	procedure f: unit -> void

Now, procedures cannot be used 'in' expressions anywhere,
provided we do not allow the argument of a function or procedure
to have 'void' type. We can also treat procedures uniformly
from a typing view point.


John (Max) Skaller,
10/1 Toxteth Rd Glebe NSW 2037 Australia voice: 61-2-9660-0850
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