Include Extern specification

grammars/Quil.g4

@@ -29,6 +29,8 @@ instr               : gate
                     | include
                     | pragma
                     | memoryDescriptor
+                    | extern
+                    | call
                     ;
 
 // C. Static and Parametric Gates
@@ -118,6 +120,13 @@ include             : INCLUDE STRING ;
 pragma              : PRAGMA IDENTIFIER pragma_name* STRING? ;
 pragma_name         : IDENTIFIER | INT ;
 
+
+// N. Declaring and Calling External Functions
+
+extern              : EXTERN IDENTIFIER ;
+call                : CALL IDENTIFIER call_arg+ ;
+call_arg            : addr | number ;
+
 // Expressions (in order of precedence)
 
 expression          : LPAREN expression RPAREN                  #parenthesisExp
@@ -131,7 +140,7 @@ expression          : LPAREN expression RPAREN                  #parenthesisExp
                     | addr                                      #addrExp
                     ;
 
-function            : SIN | COS | SQRT | EXP | CIS ;
+function            : SIN | COS | SQRT | EXP | CIS | IDENTIFIER;
 sign                : PLUS | MINUS ;
 
 // Numbers

specgen/quil.lisp

@@ -229,7 +229,7 @@
   (let ((doc (make-instance 'document
                :title "Quil Specification"
                :author "Robert S. Smith; Rigetti & Co. Inc.; and contributors"
-               :version "2021.1 (DRAFT)"
+               :version "2024.1 (DRAFT)"
                :body (append
                       (include (spec/ "sec-intro.s"))
                       (include (spec/ "sec-opsem.s"))

specgen/spec/sec-other.s

@@ -22,11 +22,120 @@ semantics.}
     PRAGMA @ms{Identifier} @rep{@group{@ms{Identifier} @alt @ms{Integer}}} @rep[:max 1]{@ms{String}}
 }
 
+
+@subsection[:title "Extern Functions"]
+
+@p{Programmers and researchers may wish to avail themselves of a rich
+vocabulary of analytic and stochastic functions in the designs of
+their experiments and algorithms. Quantum control systems or quantum
+simulators that consume Quil code may support a variety of functions
+that operate on classical data. Quil addresses these cases by
+supporting the declaration of extern functions.}
+
+@subsubsection[:title "Declaring Externs"]
+
+@p{Declaring an identifier to be an extern indicates that the
+identifier can appear in call instructions.}
+
+@syntax[:name "Extern Function Declaration"]{
+    EXTERN @ms{Identifier}
+}
+
+@subsubsection[:title "Extern Signature Pragmas"]
+
+@p{Under some circumstances it may be desirable to specify the
+function signature of an external function. Signature declarations are
+supplied by way of a special pragma.}
+
+@syntax[:name "Extern Signature Pragma"]{
+    PRAGMA EXTERN @ms{Identifier} "@ms{Extern Signature}"
+}
+
+@syntax[:name "Extern Signature"]{
+    @rep[:min 0 :max 1]{@ms{Base Type}} ( @ms{Extern Parameter} @rep[:min 0]{@group{ , @ms{Extern Parameter} }} ) 
+}
+
+@syntax[:name "Extern Parameter"]{
+    @ms{Identifier} : @rep[:min 0 :max 1]{mut} @ms{Type}
+}
+
+@subsubsection[:title "Call Instructions"]
+
+@p{Declared externs may be appear in call instructions.  The precise
+meaning of an extern call is left up to the implementor.  From the
+perspective of the abstract machine, a call to an external function
+increments the program counter and has some effect on classical
+memory.}
+
+@syntax[:name "Extern Call Instruction"]{
+    CALL @ms{Identifier} @rep[:min 1]{@group{@ms{Identifier} @alt @ms{Memory Reference} @alt @ms{Complex}}}
+}
+
+@p{When a supplied function type signature specifies a return type,
+then calls to the associated extern are assumed to write their return
+value to a memory reference passed in at the first argument position.
+E.g. the following are equivalent:}
+
+@clist{
+PRAMGA EXTERN rng "INTEGER (seed : INTEGER)"
+EXTERN rng;
+DECLARE num INTEGER
+... snip ...
+
+CALL rng num 10
+}
+
+@p{is roughly the same as}
+
+@clist{
+PRAMGA EXTERN rng "(out : mut INTEGER, seed : INTEGER)"
+EXTERN rng;
+DECLARE num INTEGER
+... snip ...
+
+CALL rng num 10
+}
+
+
+@subsubsection[:title "Externs in Arithmetic Expressions"]
+
+@p{Extern calls may appear in arithmetic expressions with some
+restrictions: the extern MUST have a known function signature, which
+MUST include a return type, and which MUST NOT include any mutable
+parameters.}
+
+@p{For example, this is OK:}
+
+@clist{
+PRAMGA EXTERN prng "REAL (seed : REAL)"
+EXTERN prng;
+RX(prng(pi) / 4)
+}
+
+@p{But this is not:}
+
+@clist{
+
+PRAGMA EXTERN irng "INTEGER (seed : mut INTEGER)"
+EXTERN irng;
+EXTERN prng;
+DECLARE num INTEGER;
+
+... snip ...
+
+RX(irng(num))       # WRONG: the seed parameter is declared mutable
+
+RZ(prng(33))        # WRONG: we don't know the signature of prng
+
+}
+
+
 @subsection[:title "File Inclusion"]
 
 @p{One can include a valid Quil file in another valid Quil file by
 inclusion.}
 
+
 @syntax[:name "File Include"]{
     INCLUDE @ms{String}
 }

specgen/spec/sec-structure.s

@@ -125,7 +125,7 @@ i pi
 ADD AND AS CONTROLLED CONVERT DAGGER DECLARE DEFCIRCUIT DEFGATE DIV EQ
 EXCHANGE FORKED GE GT HALT INCLUDE IOR JUMP JUMP-UNLESS JUMP-WHEN
 LABEL LE LOAD LT MATRIX MEASURE MOVE MUL NEG NOP NOT OFFSET PAULI-SUM
-PERMUTATION PRAGMA RESET SHARING STORE SUB WAIT XOR
+PERMUTATION PRAGMA RESET SHARING STORE SUB WAIT XOR EXTERN CALL
 }
 }
 
@@ -252,6 +252,7 @@ object, like classical memory registers.}
      @ms{Gate Definition}
 @alt @ms{Circuit Definition}
 @alt @ms{Classical Memory Declaration}
+@alt @ms{Extern Function Declaration} @ms{Terminator}
 }
 
 @p{A @emph{directive} specifies information to software processing
@@ -270,6 +271,7 @@ Quil, such as the @quil{INCLUDE} directive for including files.}
 @alt @ms{Measurement Instruction}
 @alt @ms{Circuit Application}
 @alt @ms{Classical Memory Instruction}
+@alt @ms{Extern Call Instruction} 
 @alt @ms{Reset Instruction}
 @alt @ms{Wait Instruction}
 @alt @ms{Branch Instruction}