We have quite a lot of legacy XPath expressions and are considering to migrate the code over to Groovy + XmlSlurper + GPath.
I'm thinking the task would be smoother if there was a tool (or an API) that takes an existing XPath and produces/executes the corresponding GPath. I.e. something like:
def resp = new XmlSlurper().parseText(responseAsXml)
def gpath = GPathSomething.compile("//foo/bar[id='123']")
def result = gpath.execute(resp)
def result2 = gpath.'**'.grep( it.#bar = 'baz' )
P.S. Of course, the same can be achieved by straightforward application of XPath APIs, but it feels more right to stick with native way of doing things (GPath).
P.P.S. On the other hand, may be GPath is not a good choice? Would appreciate any comments.
No ready to use product found. Made a decision to go with XPath, and call Java XPath API (Saxon) from Groovy. Works good.
Related
I dont understand how #{...} construct is used in Ruby.
I've seen in used in the regexp example on http://www.ruby-doc.org/core-1.9.3/Regexp.html
place = "tokyo"
/#{place}/.match("Go to tokyo")
#=> #<MatchData "tokyo">
What exactly is this #{...} feature called and does anyone know of some good working examples of this.
Really appreciate the help.
Thanks!
Here's an example that's a bit simpler:
place = "Tokyo"
puts "Go to #{place}"
What the #{...} construct does is to execute the ruby code that it contains, and return a string representation of the result, which then is embedded in the string where the construct appears.
Another example:
place = "Tokyo"
puts "#{place} is a #{place.class} of #{place.length} characters"
In other words, your example is equivalent to:
/tokyo/.match("Go to tokyo")
Hope this helps.
Is called interpolation, and allows you to convert placeholders to the value they represent...
http://kconrails.com/2010/12/08/ruby-string-interpolation/
The #{...} is especially useful and used quite a lot in metaprogramming. It helps you to dispatch methods dynamically without knowing the name of these methods before run time.
if conf.rc and File.exists?( conf.rc )
YAML.load_file(conf.rc).each do |k,v|
conf.send("#{k}=" , v)
end
end
As you can see, until run time we do not know which methods are going to be dispatched. Through .send and #{...}, we can dynamically dispatch methods. For example, in above code depending on the values in conf.rc different methods can be dispatched.
Example is taken from Metaprogramming Ruby.
I am using open4::background to open a process on the shell. open4::background allows me to use any class that implements <<, each, read, or to_s as a handle to stdx.
I am currently using a String, but every time a write occurs on stdout, it replaces the old value. Is there a simple class I can use to make new write append instead of replace, considering the acceptance of any class that implements certain string-like functions?
I am new to Ruby and I am just hoping to plug this part in. If anyone can contribute or point me to a simple existing class for this I'd appreciate it.
There is a thread explaining this with a sample implentation here: http://www.ruby-forum.com/topic/151316 but I think that is a bit too complex for what I'm looking to do now. As a Ruby n00b, I'd feel more comfortable if someone else could massage that sample for me.
Thanks.
EDIT:
Per Phrogz's request, here is what I want to be able to do:
app_str = AppendedString
app_str = 'jeff'
app_str = 'walls'
puts app_str
# should display "jeffwalls"
What you want is to use StringIO to capture all the contents as a string for you.
(As #John says, repeated assignment you can't catch, but if you just want to accumulate all the values created by << then this will do it.)
app_str = AppendedString
app_str = 'jeff'
app_str = 'walls'
This would require overriding the assignment operator. Unfortunately, Ruby doesn't permit overriding the assignment operator.
I have been messing about with making a sortable table module thing. I know some might exist but want to get experience doing this myself. I had the idea of have it like so:
SortedTable.new(ModelName, Hash_Of_Columns_And_Fields, ID)
example
SortedTable.new(Post, {"Title" => "title", "Body" => "body", "Last Comment" => "comment.last.title"}, params[:id])
I am planning to do something like:
def initialize(model, fields, id)
data = {}
model = model.capitalize.constantize
model.find(id)
fields.each do |column, field|
data[column] = model.send(field)
end
end
This works fine for title and body but when it comes to getting the Last Comment with comment.last.title it errors out. I have tried doing Post.send("comments.last.title") but says NoMethodError: undefined method 'comments.last.title' for #<Post:0x0000010331d220>
I know I can do Post.send("comments").send("last").send("title") and that works but I can not think of how to do that dynamically by taking the fields and spliting the on . then chaining the sends. Can anyone give me advice on how to do this? If I am doing this completely wrong also then please say or point me in the direction of code that does something similar. I am not a expert ruby developer but I am trying.
P.S The above code might not work as I am not at a computer with ruby/rails to test, but hopefully you get the concept.
Cheers
first and dirtiest solution is eval
fields.each do |column, field|
data[column] = eval("#{model}.#{field}")
end
next solution little more functional
fields.each do |column, field|
data[column] = field.split(".").inject(model){|obj, met| obj.send(met)}
end
PS
And your design is ugly
EDIT
The inject can be written more concisely as field.split('.').inject(model, :send). And I'd strongly discourage the eval way — unnecessary evals are one more case where you can slip up and allow arbitrary code execution, and they're also slow. (Also, I'm pretty sure that should just be eval("model.#{field}") — you don't want to interpolate the string value of model. Yet another example of how easy it is to slip up an eval expression.) – #Chuck
"Is 'eval' supposed to be nasty?" inspired this one:
Mostly everybody agrees that eval is bad, and in most cases there is more elegant/safer replacement.
So I wanted to ask: if eval is misused that often, is it really needed as a language feature? Is it doing more evil than good?
Personally, the only place I find it useful is to interpolate strings provided in config file.
Edit: The intention of this question is to get as many real-life cases as possible when eval is the only or the best solution. So please, don't go into "should a language limit a programmer's creativity" direction.
Edit2: And when I say eval, of course I refer to evaling string, not passing ruby block to instance_eval or class_eval.
The only case I know of (other than "I have this string and I want to execute it") is dynamically dealing with local and global variables. Ruby has methods to get the names of local and global variables, but it lacks methods to get or set their values based on these names. The only way to do AFAIK is with eval.
Any other use is almost certainly wrong. I'm no guru and can't state categorically that there are no others, but every other use case I've ever seen where somebody said "You need eval for this," I've found a solution that didn't.
Note that I'm talking about string eval here, by the way. Ruby also has instance_eval, which can take either a string or a block to execute in the context of the receiver. The block form of this method is fast, safe and very useful.
When is it justified? I'd say when there's no reasonable alternative. I was able to think of one use where I can't think of an alternative: irb, which, if you dig deep enough (to workspace.rb, around line 80 in my copy if you're interested) uses eval to execute your input:
def evaluate(context, statements, file = __FILE__, line = __LINE__)
eval(statements, #binding, file, line)
end
That seems pretty reasonable to me - a situation where you specifically don't know what code you're going to have to execute until the very moment that you're asked to do so. Something dynamic and interactive seems to fit the bill.
The reason eval is there is because when you need it, when you really need it, there are no substitutes. There's only so much you can do with creative method dispatching, after all, and at some point you need to execute arbitrary code.
Just because a language has a feature that might be dangerous doesn't mean it's inherently a bad thing. When a language presumes to know more than its user, that's when there's trouble.
I'd argue that when you find a programming language devoid of danger, you've found one that's not very useful.
When is eval justified? In pragmatic terms, when you say it is. If it's your program and you're the programmer, you set the parameters.
There is one very important use-case for eval() which cannot (AFAIK) be achieved using anything else, and that is to find the corresponding object reference for a binding.
Say you have been passed a block but (for some reason) you need access to object context of the binding, you would do the following:
obj = eval('self', block.binding)
It is also useful to define the following:
class Proc
def __context__
eval('self', self.binding)
end
end
IMO mostly for Domain Specific Languages.
"Evaluation Options in Ruby" is an article by Jay Fields about it on InfoQ.
eval is a tool, it is neither inherently good nor evil. It is justified whenever you are certain it is the right tool for what you are trying to accomplish.
A tool like eval is about evaluating code at runtime vs. "compile" time. Do you know what the code is when you launch Ruby? Then you probably don't need eval. Is your code generating code during runtime? then you probably need to eval it.
For example, the methods/functions needed in a recursive decent parser depend on the language being parsed. If your application builds such a parser on-the-fly, then it might make sense to use eval. You could write a generalized parser, but it might not be as elegant a solution.
"Programatically filling in a letrec in Scheme. Macros or eval?" is a question I posted about eval in Scheme, where its use is mostly unavoidable.
In general eval is a useful language feature when you want to run arbitrary code. This should be a rare thing but maybe you are making your own REPL or you want to expose the ruby run-time to the end user for some reason. It could happen and that is why the feature exists. If you are using it to work around some part of the language (e.g. global variables) then either the language is flawed or your understanding of the language is flawed. The solution is typically not to use eval but to either better understand the language or pick a different language.
It's worth noting that in ruby particulary instance_eval and class_eval have other uses.
You very likely use eval on a regular basis without even realizing it; it's how rubygems loads the contents of a Gemspec. Via rubygems/lib/specification.rb:
# Note: I've removed some lines from that listing to illustrate the core concept
def self.load(file)
code = File.read(file)
begin
_spec = eval code, binding, file # <-------- EVAL HAPPENS HERE
if Gem::Specification === _spec
return _spec
end
warn "[#{file}] isn't a Gem::Specification (#{_spec.class} instead)."
rescue SignalException, SystemExit
raise
rescue SyntaxError, Exception => e
warn "Invalid gemspec in [#{file}]: #{e}"
end
nil
end
Typically, a gem specification would look like this:
Gem::Specification.new do |s|
s.name = 'example'
s.version = '0.1.0'
s.licenses = ['MIT']
s.summary = "This is an example!"
s.description = "Much longer explanation of the example!"
s.authors = ["Ruby Coder"]
s.email = 'rubycoder#example.com'
s.files = ["lib/example.rb"]
s.homepage = 'https://rubygems.org/gems/example'
s.metadata = { "source_code_uri" => "https://github.com/example/example" }
end
Note that the gemspec file simply creates a new object but does not assign it nor send it anywhere.
Trying to load or require this file (or even executing it with Ruby) will not return the Gem::Specification value. eval is the only way to extract the value defined by an external ruby file.
One use of eval is compiling another language to ruby:
ruby_code = "(def foo (f a b) (mapv f (cons a b)))".compile_to_ruby
# "foo_proc = ->(f a b) { mapv_proc.call(f, (cons_proc.call(a, b)) }"
eval ruby_code
I use a 3D modeling software that implemented Ruby for writing custom text macros. In that software we are given access to model data in the form of name:value pairs accessed using the following format:
owner.name
#=> value
So for a 36 inch tall cabinet, I could access the height and convert its value to feet like so:
owner.height.to_f / 12
The main problem is that objects in that software have no unique identifiers aside from something called their schedule_number. If I want to name a variable using the schedule_number in the variable name so that I can call and use that value elsewhere, the only possible way I know to do that is by using eval:
eval "#{owner.schedule_number} = owner.height"
I'm interested in building a DSL in Ruby for use in parsing microblog updates. Specifically, I thought that I could translate text into a Ruby string in the same way as the Rails gem allows "4.days.ago". I already have regex code that will translate the text
#USER_A: give X points to #USER_B for accomplishing some task
#USER_B: take Y points from #USER_A for not giving me enough points
into something like
Scorekeeper.new.give(x).to("USER_B").for("accomplishing some task").giver("USER_A")
Scorekeeper.new.take(x).from("USER_A").for("not giving me enough points").giver("USER_B")
It's acceptable to me to formalize the syntax of the updates so that only standardized text is provided and parsed, allowing me to smartly process updates. Thus, it seems it's more a question of how to implement the DSL class. I have the following stub class (removed all error checking and replaced some with comments to minimize paste):
class Scorekeeper
attr_accessor :score, :user, :reason, :sender
def give(num)
# Can 'give 4' or can 'give a -5'; ensure 'to' called
self.score = num
self
end
def take(num)
# ensure negative and 'from' called
self.score = num < 0 ? num : num * -1
self
end
def plus
self.score > 0
end
def to (str)
self.user = str
self
end
def from(str)
self.user = str
self
end
def for(str)
self.reason = str
self
end
def giver(str)
self.sender = str
self
end
def command
str = plus ? "giving ##{user} #{score} points" : "taking #{score * -1} points from ##{user}"
"##{sender} is #{str} for #{reason}"
end
end
Running the following commands:
t = eval('Scorekeeper.new.take(4).from("USER_A").for("not giving me enough points").giver("USER_B")')
p t.command
p t.inspect
Yields the expected results:
"#USER_B is taking 4 points from #USER_A for not giving me enough points"
"#<Scorekeeper:0x100152010 #reason=\"not giving me enough points\", #user=\"USER_A\", #score=4, #sender=\"USER_B\">"
So my question is mainly, am I doing anything to shoot myself in the foot by building upon this implementation? Does anyone have any examples for improvement in the DSL class itself or any warnings for me?
BTW, to get the eval string, I'm mostly using sub/gsub and regex, I figured that's the easiest way, but I could be wrong.
Am I understanding you correctly: you want to take a string from a user and cause it to trigger some behavior?
Based on the two examples you listed, you probably can get by with using regular expressions.
For example, to parse this example:
#USER_A: give X points to #USER_B for accomplishing some task
With Ruby:
input = "#abe: give 2 points to #bob for writing clean code"
PATTERN = /^#(.+?): give ([0-9]+) points to #(.+?) for (.+?)$/
input =~ PATTERN
user_a = $~[1] # => "abe"
x = $~[2] # => "2"
user_b = $~[3] # => "bob"
why = $~[4] # => "writing clean code"
But if there is more complexity, at some point you might find it easier and more maintainable to use a real parser. If you want a parser that works well with Ruby, I recommend Treetop: http://treetop.rubyforge.org/
The idea of taking a string and converting it to code to be evaled makes me nervous. Using eval is a big risk and should be avoided if possible. There are other ways to accomplish your goal. I'll be happy to give some ideas if you want.
A question about the DSL you suggest: are you going to use it natively in another part of your application? Or do just plan on using it as part of the process to convert the string into the behavior you want? I'm not sure what is best without knowing more, but you may not need the DSL if you are just parsing the strings.
This echoes some of my thoughts on a tangental project (an old-style text MOO).
I'm not convinced that a compiler-style parser is going to be the best way for the program to deal with the vaguaries of english text. My current thoughts have me splitting up the understanding of english into seperate objects -- so a box understands "open box" but not "press button", etc. -- and then having the objects use some sort of DSL to call centralised code that actually makes things happen.
I'm not sure that you've got to the point where you understand how the DSL is actually going to help you. Maybe you need to look at how the english text gets turned into DSL, first. I'm not saying that you don't need a DSL; you might very well be right.
As for hints as to how to do that? Well, I think if I were you I would be looking for specific verbs. Each verb would "know" what sort of thing it should expect from the text around it. So in your example "to" and "from" would expect a user immediately following.
This isn't especially divergent from the code you've posted here, IMO.
You might get some milage out of looking at the answers to my question. One commenter pointed me to the Interpreter Pattern, which I found especially enlightening: there's a nice Ruby example here.
Building on #David_James' answer, I've come up with a regex-only solution to this since I'm not actually using the DSL anywhere else to build scores and am merely parsing out points to users. I've got two patterns that I'll use to search:
SEARCH_STRING = "#Scorekeeper give a healthy 4 to the great #USER_A for doing something
really cool.Then give the friendly #USER_B a healthy five points for working on this.
Then take seven points from the jerk #USER_C."
PATTERN_A = /\b(give|take)[\s\w]*([+-]?[0-9]|one|two|three|four|five|six|seven|eight|nine|ten)[\s\w]*\b(to|from)[\s\w]*#([a-zA-Z0-9_]*)\b/i
PATTERN_B = /\bgive[\s\w]*#([a-zA-Z0-9_]*)\b[\s\w]*([+-]?[0-9]|one|two|three|four|five|six|seven|eight|nine|ten)/i
SEARCH_STRING.scan(PATTERN_A) # => [["give", "4", "to", "USER_A"],
# ["take", "seven", "from", "USER_C"]]
SEARCH_STRING.scan(PATTERN_B) # => [["USER_B", "five"]]
The regex might be cleaned up a bit, but this allows me to have syntax that allows a few fun adjectives while still pulling the core information using both "name->points" and "points->name" syntaxes. It does not allow me to grab the reason, but that's so complex that for now I'm going to just store the entire update, since the whole update will be related to the context of each score anyway in all but outlier cases. Getting the "giver" username can be done elsewhere as well.
I've written up a description of these expressions as well, in hopes that other people might find that useful (and so that I can go back to it and remember what that long string of gobbledygook means :)