Subject: publish
From: Vassily Litvinov (vass@cs.washington.edu)
Date: Thu Mar 29 2001 - 21:52:56 PST
I have published in Cecil/src/compiler. Essentially these are changes
to typechecker and parser, to
- allow polymorphic synonyms, and
- change the syntax of synonym decls (= instead of standsfor)
Baseline binaries to be installed shortly.
I tested synonyms, but watch out for bugs.
DETAILS
Synonyms are allowed to be parameterized and/or have type context
just like other polymorphic declarations:
forall F where W1: type synonym somename[T1,T2] = sometype where W2;
where [T1,T2] are synonym's formal parameters (optional).
NOTE I replaced the "standsfor" keyword (which is no longer a Cecil keyword)
with the "=" operator symbol.
DESUGARING RULES:
When such synonym is used (e.g. as in somename[int,string]),
- the "sometype" gets syntactically replaced by substituting
the actual type parameters (here int, string) for the formals
(here T1, T2)
- the synonym's other type variables (aka "implicit" type parameters) are
replaced by fresh type variables added to the enclosing declaration's
"forall" clause
- the synonym's constraints are added (after the above substitutions)
to the enclosing declaration's "where" clause.
In method bodies, (global) variable declarations or field initializers
reference to synonyms with constraints and/or implicit type variables
is disallowed.
For example, given the following
forall Key, Value where Value <= num:
type synonym table[Value] = table[Key,Value];
method foo(x:table[`T]):void { ... }
The header of method foo will be desugared into:
forall T, Key' where `T <= num: method foo(x:table[Key,T]):void { ... }
Referring to table[T] or table[int] inside the elipses generates type error.
BACKQUOTE SUGAR is supported by treating the substitutions as purely
syntactic. For example, given
type bounded[T1, T2, T3] where signature bound(T1,T2,T3):void;
type synonym bsyn[Q] = bounded[Q, int, `R];
(where `R is the synonym's implicit type parameter), bsyn desugars as:
bsyn[T] -> bounded[T, int, `R]
bsyn[`T] -> bounded[`T, int, `R]
bsyn[string] -> bounded[string, int, `R]
BACKQUOTE SUGAR ON NAMED TYPES
While I am on this, I'll explain this one. Assume named type declaration:
forall T1, T2, T3 where W: type mytype[T1,T2];
When an instance of "mytype" occurs inside the header of a polymorphic
declaration and (at least) one of the actuals of "mytype" is a type
variable with backquote (as in mytype[int,`Q]), that header is
extended with fresh variables corresponding to mytype's implicit
parameters (T3 in this case) and those constraints from W that
reference mytype's formals whose actuals have backquotes (after
appropriate substitutions).
For example, if the header references mytype[`Q1,Q2], then the
declaration is extended with T3' and those constraints from W that
reference T1 or T3, upon substitution T1<-Q1, T2<-Q2, T3<-T3'.
Outside of polymorphic declaration headers, backquotes are not allowed
and therefore declarations are not extended.
-Vass
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