Add constraint synonyms

Currently not enabled by default, the flag -Xconstraint_synonyms
enables them

For generating constraints in ASL parser, we want to be able to give
names to the constraints that we attach to certain variables. It's
slightly awkward right now when constraints get long complicated
because the entire constraint always has to be typed out in full
whenever it appears, and there's no way to abstract away from that.

This adds constraint synonyms, which work much like type synonyms
except for constraints, e.g.

constraint Size('n) = 'n in {1, 2, 4, 8} | 128 <= 'n <= 256

these constraints can then be used instead of the full constraint, e.g.

val f : forall 'n, where Size('n). int('n) -> unit

Unfortunatly we need to have a keyword to 'call' the constraint
synonym otherwise the grammer stops being LR(1). This could be
resolved by parsing all constraints into Parse_ast.atyp and then
de-sugaring them into constraints, which is what happens for
n-expressions already, but that would require quite a bit of work on
the parser.

To avoid this forcing changes to any other parts of Sail, the intended
invariant is that all constraints appearing anywhere in a type-checked
AST have no constraint synonyms, so they don't have to worry about
matching on NC_app, or calling Env.expand_typquant_synonyms (which
isn't even exported for this reason).

1 files changed, 5 insertions, 1 deletions

diff --git a/src/parser.mly b/src/parser.mly
index b9aae275..070dee50 100644
--- a/src/parser.mly
+++ b/src/parser.mly
@@ -175,7 +175,7 @@ let rec desugar_rchain chain s e =
 
 /*Terminals with no content*/
 
-%token And As Assert Bitzero Bitone By Match Clause Dec Default Effect End Op
+%token And As Assert Bitzero Bitone By Match Clause Dec Default Effect End Op Tuple Where
 %token Enum Else False Forall Foreach Overload Function_ Mapping If_ In Inc Let_ Int Order Cast
 %token Pure Register Return Scattered Sizeof Struct Then True TwoCaret TYPE Typedef
 %token Undefined Union Newtype With Val Constraint Throw Try Catch Exit Bitfield
@@ -331,6 +331,8 @@ nc_and:
     { $1 }
 
 atomic_nc:
+  | Where id Lparen typ_list Rparen
+    { mk_nc (NC_app ($2, $4)) $startpos $endpos }
   | True
     { mk_nc NC_true $startpos $endpos }
   | False
@@ -1402,6 +1404,8 @@ def:
     { DEF_scattered (mk_sd (SD_scattered_end $2) $startpos $endpos) }
   | default_def
     { DEF_default $1 }
+  | Constraint id Lparen kid_list Rparen Eq nc
+    { DEF_constraint ($2, $4, $7) }
   | Mutual Lcurly fun_def_list Rcurly
     { DEF_internal_mutrec $3 }
   | Pragma