chapter-06.srt

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Chapter 06.
Raising an exception when a build step fails.

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JC: The thing that gets trickier about
this is when you have multiple instructions.

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You want them all to be executed

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but because they have side-effects you
want them done in a particular order.

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Specifying that with mocks
can be a bit tricky but...

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One thing that we can do, which is a use-case
I've not got to yet: what if any of them fails?

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So we can say something like: if the first one fails,

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the second one should not be executed
and that implies that there's an ordering.

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So let's do that context:
'when there are many instructions'

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let the instructions be
'first-instruction' and 'second-instruction'.

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Again, it doesn't matter what these are
because we're not going to execute them.

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'it prints all the instructions'.

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Oh no, expect output.

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With 'first-instruction' once.

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And also the second instruction.

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When we build the target.

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And we also want to execute all of the instructions.

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That should be the target in this case.

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And puts should be system when you build target.

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I believe these will work.

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They do.
TS: Great

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JC: Now we want to say
'when the first instruction fails'.

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TS: So you're saying here that it doesn't matter
what order it does it, that's not testing it?

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JC: No, RSpec will not check.

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RSpec doesn't expect things to be called
in the order in which you set up the mocks.

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You could switch these lines around.

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If I switch those this will still pass.

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Even though that isn't what I want.

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But specifying that you want
things called in a certain order is...

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The way that I program is an uncommon use
case and is awkward to specify using mocks.

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But if you have something where one thing
can prevent the next thing from happening,

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then that implies an ordering.  It's enough to
say that one thing happens before the other.

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TS: And that's something else
that you want to test anyway.

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JC: Yes you want it to abort the
build if something bad happens.

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So when the first instruction fails, it
means that I'm going to allow the target

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to receive system with
'first-argument' and return nil.

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Which is what system returns
if the command doesn't work.

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So that scenario...(I'm not going to bother with
the printing stuff, we've done that to death).

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It does execute the first argument.

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But it does not execute the second.

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So that means we expect this... Once.

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[Silence]

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Once.

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Do we need to tell it that it will
return nil? Not sure that we do.

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We'll find out.

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I think I've done something weird
to Vim. Oh, I've got caps lock on, sorry!

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I'll copy that.

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So let me find the second-
they're not arguments, they're instructions.

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And it's not 'once', it's 'not'.

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So let me check this over.

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So if the target returns nil
for the first instruction,

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it should still execute it, but not the second one.

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This shouldn't work because I haven't
implemented anything like this yet.

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So expect it to use a target
(that I haven't actually told it to build).

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That's quite important. [laughter].

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target.build

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[Silence]

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Now it's executing the second
instruction when it shouldn't.

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So if we go back to our code we can
say it's going to print the instruction.

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And then we could raise failed command
unless system returns something truthy.

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TS: So what will system return if it's not an error?

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JC: True. We can see this in the shell for a second.

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So if you system, echo something,
it does the thing and returns true.

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And if you ask it to run a program
that doesn't exist it returns nil.

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TS: OK great.

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JC: So it's quite handy for having a high
level API onto what your system is doing.

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So make an exception and raise that
unless system returns something truthy.

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TS: So you've made a decision here that you don't want
it to skip them silently, you want them to actually...

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JC: No because a build is just a sequence.

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You turn a CoffeeScript file into a JavaScript file
and you need to put multiple files into a single one.

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If the first step fails you can't do the second step.

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And you shouldn't try
because if those files don't exist,

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like if your command uses a shell wildcard or
tries to interpolate something that doesn't exist,

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you might end up constructing a
command that will do something dangerous.

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So it really ought to check that everything
you've done so far did what you expected.

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So we shall run that. OK so loads of things are
now not working, which is common, and fine.

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So let's find out why that was.

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A lot of these raised, I was not
expecting them to be raised.

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For example, the first one, 33 lets run that.

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It's raising,

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And it's raising from build.

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We said we're going to raise unless
system returned something truthy.

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But we haven't made system
return anything, we stubbed it out.

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TS: So that will be returning nil by default.

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JC: Yes, so we should make that return true and
anywhere else we're saying it should be called.

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That's a not so it doesn't matter.

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TS: Earlier you said you were expecting these
to inherit the behaviour of the stub anyway.

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JC: That's a good point yes.

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I think if you stub something and
then put mock expectation on it,

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the thing at the top says if you call
it with anything it just returns true.

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The mocks tend to be more specific.

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Either they're saying not to but it doesn't
matter because it's not going to be called.

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But here we're saying it should be called
with something and that's a different use case.

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I believe in those situations you have
to tell it again what you want it to return.

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That's passing, a couple are
still not passing 64 and 69.

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[Silence]

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TS: Why were they failing?

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JC: I believe this will raise and it's supposed
to raise, but I'm not catching that it raises.

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I should have another test that
says it raises FailedCommand.

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Expect target.build to raise
error and FakeMake FailedCommand.

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TS: So the problem with these tests is that
the exception was bubbling out of the tests.

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JC: Right.

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And then I can just rescue in here.

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[Silence]

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OK so that's all green again.

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TS: Great.

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JC: It took a while to get there,

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But I think that's enough of a specification
of what the thing does about running commands.

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We've now got a thing that can run arbitrary commands
and can do something reasonably sensible with them.

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So I'm going to get out of here to make a commit.

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So what have we got?

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I might just move build.sh to my scratch
directory and keep that hanging around.

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So I ought to add the Gemfile things and the
example, although not everything in example.

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There are a few things we definitely don't want.

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I don't want an example bundle.js or example lib.

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Nor node modules, because they
are just for running the examples.

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I don't want scratch.

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I don't want package.json either, we're
building a Ruby project – although that is...

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No that's metadata about the examples
and you will use it to install your node modules.

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But you don't want node modules... that's fine.

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And we moved build.sh out of there. So if I
git add all of this. So what do we end up with?

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Git ignore and some gem files and some example code

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and our source code and package.json and specs.

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Which looks reasonable to me.

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[Silence]

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OK.