Reference parameters

You have already met reference parameters as an end-user. When in a command you issue an instruction like get data from File("filename"), then data is not playing the ordinary role of a parameter in a command, to be evaluated and then passed. Instead, the variable data itself is being passed. The signature of the command is get (x ref) from (file File), where the ref in the signature tells Pipefish to take the variable itself, and not its value, as its argument.

Let's have some simple examples as found in the first few lines of examples/ref.pf:

var

z = 42

cmd

zero (x ref) :
    x = 0

(x ref) ++ :
    x = x + 1

If we try it out in the REPL:

→ hub run "examples/ref.pf" as "REF"                                                
Starting script 'examples/ref.pf' as service 'REF'.
REF → z
42
REF → zero z 
ok
REF → z 
0
REF → z ++ 
ok
REF → z 
1 

People experienced with this sort of idiom might ask how, if we wanted to, we would prevent the xs in the example commands from dereferencing themselves. You can't: why would you ever want to? The only case in which they don't is when the reference parameter is passed to another command which is expecting a reference, as shown here.

doThing(x ref) :
    zero x 
    x ++
    post x

Only commands have reference parameters; functions don't because the only reason you would want to pass a reference instead of its value is so you could mutate it: but functions have no way of mutating anything, so a ref in a function definition would be superfluous and misleading.