Flow of control

Pipefish commands have flow of control in a way that Pipefish functions don't and can't. This is demonstrated in the file examples/flow.pf.

Let's look at the commands in it one at a time.

Commands are sequences of instructions

Unlike a Pipefish function, which just evaluates things, Pipefish commands do things, and so can and indeed must do them one after another.

seq :
    post "It's just one thing ..." to Output()
    post "... after another." to Output()

Conditionals

And so this means that in Imperative Pipefish one conditional can follow on another:

check :
    5 % 2 == 0 :
        post "5 is even" to Output()
    else :
        post "5 is odd" to Output()
    6 % 2 == 0 :
        post "6 is even" to Output()
    else :
        post "6 is odd" to Output()

It is recommended that every conditional block should end with an else clause. First, for consistency and second because when one conditional block follows on another like this, Pipefish identifies the end of the first conditional block with the else clause.

OK

If you want one branch of a conditional to do nothing at all, then you should tell it to return OK: e.g. the following rewrite of the check command above will do nothing unless 5 becomes even or 6 becomes odd:

check :
    5 % 2 == 0 :
        post "5 is even" to Output()
    else :
        OK
    6 % 2 == 0 :
        OK
    else :
        post "6 is odd" to Output()

for loops

In Imperative Pipefish a for loop can contain statements rather than expressions.

cmd

hello(n) : 
    for i = 0; i < n; i + 1 : 
        post "Hello!"

Note that this particular for loop neither has nor needs bound variables, although they can if you wish, when needed. As the loop body is imperative, it will either return OK, which you needn't concern yourself with; or an error, which you can deal with as explained in the page on imperative error-handling.