Imperative error handling

Pipefish commands don't return values. But everything can return an error! The way Imperative Pipefish works with this is demonstrated in examples/try.pf.

try

The try keyword acts as a condition that succeeds if an error is not returned:

tryWithoutCapture :
    try :
        get x from File("non-existent file")
    else :
        post "Well, that didn't work."

try with capture

If try is followed by the name of a variable, and an error value is returned, then the error will be assigned to that variable.

tryWithCapture :
    try e :
        get x from File("non-existent file")
    else :
        post "Well, that didn't work. Error was: " + e[errorMessage]

Note that the error can only be assigned to a local variable: it is never possible to assign an error value to a global variable by this or any other means.

try as a conditional

In other respects try works like an ordinary condition. For example, it doesn't have to be the first condition in its block:

dividePotentialNulls(a, b int?) :
    a in null or b in null :
        post NULL to Output()
    try :
        post a / b to Output()
    else :
        error "tried to divide by zero"