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--- language:types2 [2024/07/18 16:28] – [Macros and Functions] rajit
+++ language:types2 [2025/09/19 10:22] (current) – [Functions] rajit
@@ Line 160: / Line 160: @@
 adder and five-bit adder.
+==== Default parameters ====
-===== Direction flags and user-defined types =====
+When defining complex user-defined types with many parameters, it can be useful to have
+default parameter values. ACT has syntax to support default parameter values for trailing
+parameters in a template definition.
+<code act>
+template <pint N; pbool active_high = true>
+defproc driver(bool? inp; bool! outp)
+{
+  bool sig;
+  prs {
+    inp => sig-
+  }
+  [active_high -> prs { sig => outp- }
+  [] else -> sig = outp;
+  ]
+}
+</code>
+(Note: this is not a real signal driver, but the idea here is the you have a parameterized
+driver that can drive a fanout of ''N'' gates.) This definition has a default value for the
+''active_high'' parameter as ''true''.  So an instance
+<code act>
+driver<4> x;
+</code>
+will have four production rules:
+<code act>
+ x.inp -> sig-
+~x.inp -> sig+
+ sig -> x.outp-
+~sig -> x.outp+
+</code>
+However, this behavior can be changed by using:
+<code act>
+driver<4,false> x;
+</code>
+In this case, ''sig'' will be connected to ''x.outp''.
+Note that ACT is very strict about type-checking; so, for example, ''driver<4>'' and ''driver<4,true>'' are //not// treated as the same type even though the default parameter value for the second template parameter is ''true''.
+==== Grouping parameters ====
+Parameters can be combined into [[language:types2:data#parameter_structures|parameter structures]], to organize a large number of parameters and treat them as a group. Parameter structures can also be used as a template parameter types.
+===== Direction flags  =====
@@ Line 205: / Line 248: @@
 is detailed in the section on connections.
-===== Macros and Functions =====
+===== Macros and Functions within User-defined types =====
 User-defined types support additional methods (beyond the special ones for channels and data types).
@@ Line 213: / Line 256: @@
 ==== Macros ====
+A macro is, as the name sounds, a CHP fragment. This fragment is used to substitute for the macro in the places where it is used. Macros can be defined for any user-defined type (except for parameter structures).  As an example, consider a process that implements a data structure like a stack.
+<code act>
+defproc stack (chan(int)? in; chan(int)!out )
+{
+  ...
+}
+</code>
+In ordinary circumstances, one would instantiate a copy of the stack, and then use the ''in'' and ''out'' channels to push or pop elements from the stack as follows:
+<code act>
+...
+stack s;
+int x;
+...
+chp {
+   ...
+   s.in!5; // push value 5
+   ...
+   s.pop?x; // pop x out of the stack
+   ...
+}
+</code>
+As an alternative, the user could provide //macros// to push and pop elements from the stack as follows:
+<code act>
+defproc stack (chan(int)? in; chan(int)!out )
+{
+  ...
+  methods {
+      macro push(int val) {
+          in!val
+      }
+      macro pop(int res) {
+          out?res
+      }
+  }
+}
+</code>
+The same stack use case can be written as:
+<code act>
+...
+stack s;
+int x;
+...
+chp {
+   ...
+   s.push(5);
+   ...
+   s.pop(x);
+   ...
+}
+</code>
 ==== Functions ====
+In addition to macros, [[language:types2:data#pure_structures|pure structures]] can also include user-defined functions. User-defined functions within pure structures have similar syntax to macros. The following is an illustrative example
+<code act>
+deftype signed_int (bool s; int<7> v)
+{
+    methods {
+        function negative() : bool
+        {
+            chp {
+              self := s
+            }
+        }
+        function mag() : int<7>
+        {
+            chp {
+               self := v
+            }
+        }
+   }
+}
+</code>
+With this definition, a user can use method calls to access the fields of the structure as follows:
+<code act>
+ signed_int s;
+ ...
+chp {
+    ...
+    [ s.negative() -> log ("Negative number!")
+    [] else -> log ("Positive number!")
+    ]
+}
+...
+</code>
+Note that functions cannot have any side-effects; in particular, this means that a function cannot change
+any of the members of the pure structure. Macros can be used to change those.
+==== Operator overloading ====
+Functions within pure structures are also used to support operator overloading. In particular, the following function methods are interpreted to be the definition of operator overloading for arithmetic operators:
+   * ''plus'', ''minus'', ''mult'', ''div'', ''mod'' : addition, subtraction, multiplication, division, and modulo operators
+   * ''uminus'' : unary minus
+   * ''and'', ''or'', ''xor'' : logical operators
+   * ''lsl'', ''lsr'', ''asr'' : logical shift left, logical shift right, arithmetic shift right
+   * ''lt'', ''gt'', ''le'', ''ge'', ''eq'', ''ne'' : comparison operators
+   * ''not'' : logical negation
+An example of a fixed-point arithmetic datatype is provided in the [[https://github.com/asyncvlsi/stdlib/blob/main/math/fxp.act|ACT standard library]], and serves as a useful reference for using operator overloading.
+In the linked example, ''fixpoint<a,b>'' corresponds to a fixed-point number with ''a'' integer bits and ''b'' fractional bits following the standard Q(a,b) format.