How to implement such a Domain-Specific-Language (DSL)?

I need to use Ruby to process performance data and calculate performance
metrics. What I have is a Hash that contains event_name => event_value
mappings. What I want to do is calculating metrics in a elegant style.

for example:

events = Hash.new

reading the raw data.

code omitted here.

now I get the basic events, like

events[“event_A”] = 123.456

events[“event_B”] = 456.789

the normal way looks like:

events[“metric_A”] = events[“event_A”] * 64 / events[“event_B”]

but what I am wondering if I can do it in an elegant way, like:

metric_A = event_A * 64 / event_B

Can anybody give some hints?

2010/12/20 Zd Yu [email protected]:

now I get the basic events, like

events[“event_A”] = 123.456

events[“event_B”] = 456.789

You may don’t need the hash at all.
Just do:

event_A = 123.456
event_B = 456.789

Then you will get what you wanted:

metric_A = event_A * 64 / event_B

Regards.

zuerrong wrote in post #969511:

You may don’t need the hash at all.

you did not get my point. I want to make it flexible. I just write the
tool, and the users of the tool can define the metric by themselves.

“Jesús Gabriel y Galán” [email protected] wrote in post
#969520:

you can do that easily with method_missing

Thanks. it looks like a promising solution. but I have two more
requirements:

  1. what if the event name includes a dot ‘.’?
  2. I also want the left value of the equation (metric_A in the example)
    to be added into the Hash, because it is possible that a new metric
    needs to be calculated based on the existing metric.

On Dec 20, 2010, at 01:18 , Zd Yu wrote:

“Jess Gabriel y Galn” [email protected] wrote in post
#969520:

you can do that easily with method_missing

Thanks. it looks like a promising solution. but I have two more
requirements:

  1. what if the event name includes a dot ‘.’?

You have two event names and you need to clarify: the key in the hash,
or the “variable” used in the DSL?

If the former, you probably want to munge the names as they come in.

If the latter, you probably want to munge what you wind up instance
eval’ing.

If neither, you’re probably doing it as a scoping mechanism of some sort
and you probably need to build a hierarchy in @events and change method
missing to do a proper hierarchical lookup (probably recursively,
passing down to subsequent sub-EventData’s).

  1. I also want the left value of the equation (metric_A in the example)
    to be added into the Hash, because it is possible that a new metric
    needs to be calculated based on the existing metric.

Then chuck the hash entirely and use local variables. This will probably
collide with the above suggestion for namespacing… so munging the
input script is a better route.

On Mon, Dec 20, 2010 at 8:22 AM, Zd Yu [email protected] wrote:

Can anybody give some hints?
If the only requirement is to have event_A evaluate to
events[“event_A”] you can do that easily with method_missing. This can
give you some ideas:

class EventData
def initialize events
@events = events
end

def method_missing meth, *args, &blk
super unless /\Aevent_/ =~ meth.to_s
@events.send(:[], meth.to_s)
end
end

data = EventData.new(“event_A” => 123.456, “event_B” => 456.789)
metric_A = data.instance_eval { event_A * 64 / event_B }

=> 17.2972291364284

You can have a file with only the block for instance_eval for your
users to type, with just an extra little bit of work.

Jesus.

“Jesús Gabriel y Galán” [email protected] wrote in post
#969520:

On Mon, Dec 20, 2010 at 8:22 AM, Zd Yu [email protected] wrote:

Can anybody give some hints?
If the only requirement is to have event_A evaluate to
events[“event_A”] you can do that easily with method_missing.

Or look at OpenStruct in the standard library (ostruct.rb), combined
with instance_eval

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