chapter-04.srt

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Chapter 4.
Sketching the API of a Target class.

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I'm just going to make a scratch
directory here and try something out.

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Yes, I'm thinking I want to do something
like this; I want to require './scratch/target'.

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And hello_js is a Target with a path.

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We are not doing Haskell,(whoops).

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The minimum... It has a path name
which is the file it's going to generate.

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and then it has a list of commands
that are how to generate that thing.

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TS: What do you mean by target here?

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JC: A target is a file (I guess I'm borrowing
some nomenclature from make here),

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but a target is any of the files in your system.

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So when you talk about hello.js is
derived from hello.coffee, hello.js is the target.

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hello.coffee could also be a
target that is derived from other things.

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They can either derived things or
primary things, like your source code.

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TS: I suppose in this case hello.js is both a target
and also it's going to be the input to the target.

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JC: Exactly. So there's a recursion there.

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And you want these things to compose in that
way so you can make targets out of other targets.

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[Silence]

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In Ruby you could do this, for example
by putting a block here. Like system... [he thinks]

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How did we do this... We ran... [he types].

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That would work, but I'm leaning toward
making this string be data rather than blocks.

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Just because it's quite common
to do, like in make you can...

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Here we've duplicated this pathname
example/src/hello.js. That's coffee isn't it.

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The idea here is that we've duplicated.
This is the directory that the target lives in.

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If you're doing make you can do that... So $@ is the
pathname of the target your currently building.

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And there's various functions so dir is the
function that gives you the directory of that.

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I'm not going to get do that yet,

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but it means that I don't necessarily want
to provide a block that just runs this code.

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I want to provide this code as data,

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because it might need to be further
interpreted by the system before being executed.

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TS: So if it's a block it's some opaque
code that can't then be modified.

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JC: Yes, I want it to be data.
So I'm going to put that in an array.

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TS: Is the expectation at this point
that you will pass that to system... Right now?

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JC: Exactly

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So if I make a target and build
it, then I want it to BE built.

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But the name of the thing I'm building and
the way in which it gets built are both data now,

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so you could put anything in it.

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TS: Why did you make it an array?

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JC: That's a good question, there's only one command.

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So we possibly don't need an array.

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There is one thing I've missed out, when I deleted
that code earlier, I left that directory out didn't I?

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So we need to make sure that directory
exists before we compile anything into it.

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So let's also make that directory part of the thing.

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You can see how this becomes repetitive because
you mention the same things over and over again.

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That ought to work from a clean slate now

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because that's all the commands that need
to be run to set up the directory and compile code.

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I want this script, when it's run, to make that file.

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[Silence]

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If I run this it will get no such file scratch/target.

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As I expected, scratch/target.rb. OK.

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So you need a Target class.

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And that's going to take the
pathname of the thing we're building.

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And some instructions.

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[Silence]

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OK

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So: undefined method build for Target... OK, that's fine.

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I want to build the thing.

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And like you said,we could pass each of those
strings into system and it ought to run them.

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So we can put @instruction.each do |instruction|

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I'm going to print what each instruction is.

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Because that's a helpful thing that make does, and it
helps you see what you build is actually doing.

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When we ran that shell script, another thing
it didn't do is show us what was going on.

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If you can see what's going on you can verify if
it's being lazy, not running the certain things.

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So we'll print each one and then pass
it to system, and that will be enough.

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I've made this `@output` variable,
partly because I'm anticipating that...

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If I'm calling puts, generally things that have
puts statements in them are not good for your tests,

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because they fill up the tests
with whatever is going on.

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It's a sort of habit I have...  If I know that
something's going to need to be a parameter

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or maybe a constant or an instance variable later.

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I'll make it but then assign it
to a constant in the constructor.

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It means I don't need to go through all the code
later and go through the extraction process.

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This may be a bit premature but I can
see that coming up so I'll just do that.

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So it's printed what it's doing and
made the directory and the coffeescript.

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And now there should be
one file in lib which is hello.js

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And that should contain compiled code, which it does.

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So I guess that's a minimum
viable build system... I suppose.

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We've got this.

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And we can make another one out of complimenter.

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[Silence]

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And...Then we could build each of them.

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And then we could have one for the bundle.

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[Silence]

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Target.new('example/bundle.js')
is what's being built here.

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And build this... What did we do in our build script?

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We ran this command. So I'll just take that.

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[Silence]

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Put that there and then...

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Build it.

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[Silence]

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Ah! This doesn't exist yet because we didn't build it.

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ruby scratch/test.rb

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OK, so that's made the lib directory twice
and run the coffee twice with the two things.

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So let's run uglify so we get
our output as we wanted it.

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This is like enough of a system, that
you can do something useful with.

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TS: OK.

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How close is this to the generalized
instructions mention in the brief?

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Are these the generalised instructions or
are they not sufficiently general to count?

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JC: They are general in that you can put any
shell command in here, and it will be run.

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There wasn't a great way for supplying the input to
the system and there's a lot that it can't do yet.

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But this is a minimal thing for
modeling a file and how it gets created.

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We're still not using the pathname input to this yet.

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That will become more important when
things want to check that they're up to date,

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because they have to know where their target is.

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This target only runs shell commands, it doesn't
look at the output of those shell commands.

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And it doesn't relate them to
the output file in any way.

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But when it wants to know if something's up to date,
it's going to have to look at those files.

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TS: I thought it was interesting that you
didn't start by building the target class,

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but by USING the target class and
then you kind of dropped down...

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And every time you ran this it failed, so you
implemented a method on it till it worked.

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JC: Right.

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I have a vague idea where I'm going because it should
look like something I'm already familiar with.

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But I don't quite know what
models and functions I want yet.

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I'm just thinking, how would
you want to interact with this?