Passing block to Proc#call


#1

I’d like to do something like:

x = lambda do |*args|
if args.size == 5
# call another block
yield *args.map{|x| x.gsub(/es$/, ‘’) }
end
end

x.call(*%w[cats dogs ones twos threes]) do |*words|
puts words.last # output: thre
end

But x.call(…) { block } raises LocalJumpError: no block given. I’m
not sure how to pass a block because it seems #call doesn’t pass it
on. Sorry if this is a common question, but I’m having a hard time
searching for help because “passing block to lamda call” appears in
every page about the basics of closures.

I see lamda {|&block| … } is added in ruby 1.9, but I’d like to be
compatible with 1.8 too.


#2

Dear Erwin,

maybe this is what you’re looking for:

def cut_off_regexp(an_array)
return proc { |regexp|
res=[]
an_array.each{|entry|
if entry.length>5 and regexp.match(entry)
res<<entry.sub(regexp,’’)
else
res<<entry
end
}
res
}
end
my_array_of_strings=[“goes”,“does”,“undoes”,“a long string”]
p1 =cut_off_regexp(my_array_of_strings)
p ‘cut off “es” at the end of long strings’
p p1.call(/es$/)
p ‘cut out “es” anywhere in a long string’
p p1.call(/es/)

I constructed this from the help about Proc in

http://www.rubycentral.com/book/tut_containers.html .

Best regards,

Axel


#3

On 10.06.2007 08:53, Erwin A. wrote:

puts words.last # output: thre
end

Why do you need x here? Do you need to pass it around? Can you show a
bit more of the picture?

But x.call(…) { block } raises LocalJumpError: no block given. I’m
not sure how to pass a block because it seems #call doesn’t pass it
on. Sorry if this is a common question, but I’m having a hard time
searching for help because “passing block to lamda call” appears in
every page about the basics of closures.

I see lamda {|&block| … } is added in ruby 1.9, but I’d like to be
compatible with 1.8 too.

There is no other compatible way than to explicitly pass a lambda / proc
as argument in 1.8.x:

irb(main):003:0> f = lambda {|a,b| 5.times { a = b[a] }; a }
=> #Proc:0x7ff7a1c8@:3(irb)
irb(main):005:0> f[0, lambda {|x| x+1}]
=> 5

Kind regards

robert


#4

On 10.06.2007 18:25, Erwin A. wrote:

match the first 3 words) with a block that processed the matching
text, and then moved the position in the string forward by 3 words. I
tried wanted to be able to do this like:

Did I understand that properly, you want to process three words at a
time and then the next three words? Then you could do

require ‘enumerator’
words.each_slice 3 do |w1,w2,w3|

end

or

words.each_slice 3 do |three_words|
patterns.each do |rxs, process|
if rxs.to_enum(:zip, three_words).all? {|rx, wd| rx =~ wd}
process[three_words]
break
end
end
end

It’s still unclear to me how exactly you want the matching to work. Are
all your “attributes” matched against all three words? Do you
positional matches? In the code all rx’s are matched against words in
the same position and if all match the block is invoked on the words.

match.call(//, /ed/, //) do {|a, b, c| “#{a} #{b.upcase} #{c}” }
Anyway, it turned out that was messy even if I didn’t have trouble
would’ve worked though the syntax isn’t as cute.

Thanks for your help Robert and Axel!

I still think you’re not yet there.

Kind regards

robert


#5

On 6/10/07, Robert K. removed_email_address@domain.invalid wrote:

x.call(*%w[cats dogs ones twos threes]) do |*words|
puts words.last # output: thre
end

Why do you need x here? Do you need to pass it around? Can you show a
bit more of the picture?

I ended up refactoring, but earlier I was parsing some text by
associating an array of attributes (like, [/a/, /b/, /c$/] that might
match the first 3 words) with a block that processed the matching
text, and then moved the position in the string forward by 3 words. I
tried wanted to be able to do this like:

match = proc do |*regexs|
if words[position…position+regexs.size] match given regexs
parsed << yield *words[ … ]
end
end

words = %w[the quick brown egg jumped under the slow zebra]
parsed = []

while position < words.size
match.call(//, /ed/, //) do {|a, b, c| “#{a} #{b.upcase} #{c}” }
match.call(/the/, //) do {|a, b| “the #{b.upcase}” }

increment position somewhere, maybe in match…? this was messy

end

parsed => [the QUICK brown egg JUMPED under the SLOW zebra]

where the matched strings were “the quick”, “egg jumped under”

and “the slow”

Anyway, it turned out that was messy even if I didn’t have trouble
passing a block. Instead I made a method scanner(*regexs, &block) that
stores everything in an array, then later another method runs through
the list and does the actual work, where it keeps track of the
position and knows if it made a match or not. Should’ve slept on it
before I mailed the list!

There is no other compatible way than to explicitly pass a lambda / proc
as argument in 1.8.x:

I forgot about passing a lambda/proc as a “normal” parameter… that
would’ve worked though the syntax isn’t as cute.

Thanks for your help Robert and Axel!


#6

On 6/10/07, Robert K. removed_email_address@domain.invalid wrote:

On 10.06.2007 18:25, Erwin A. wrote:

I ended up refactoring, but earlier I was parsing some text by
associating an array of attributes (like, [/a/, /b/, /c$/] that might
match the first 3 words) with a block that processed the matching
text, and then moved the position in the string forward by 3 words. I
tried wanted to be able to do this like:

Did I understand that properly, you want to process three words at a
time and then the next three words? Then you could do…

I already have solved the problem, but maybe someone will find this
useful in the future. Basically we start with position=0 (position is
the index of the words array). Each “match” is tried until one
succeeds, and the position is incremented by the number of words it
operated on. So if I called match.call(/the/, /quick/, /brown/, /.*/,
/e$/), it would read 5 words starting at “position” and if all the
arguments matched the words, it would process the 5 words in some way
and then increment the position by 5.

In my application I’m not really using regexs though, my words are
tokens with various tags, and I’m matching based on the tags. This is
all being used to pase date strings like “Wed Aug 5th 2008” might be
matched by a rule like match.call(:weekday, :month, :ordinal, :year)
for example. Then there might be another rule like match.call(:num,
:num, :year) that would match “05 05 2005” and would decide how to
parse it.

It’s still unclear to me how exactly you want the matching to work. Are
all your “attributes” matched against all three words? Do you
positional matches? In the code all rx’s are matched against words in
the same position and if all match the block is invoked on the words.

Basically you have it right, the words have to match their
/respective/ attribute. But it’s not a fixed number of words at a
time, because match.call(/the/) would only match one word (then
process it, then increment the position index by one).

Initially (it was late at night, mind you) I though having a closure
would work nicely because I could access position, words, and some
other variables in the caller’s scope and wouldn’t have to pass those
along every time. But it was too tricky/messy because I also needed to
restart at the beginning of the loop after a success (to start trying
all the patterns again), and I needed to know if anything had matched
(so I could increment position by 1, else have an infinite loop).

What I ended up doing was having a function to store the list of
attributes and the block that should be called to “process” the
matching words, and then another function that began scanning the word
list from position=0, testing all the attributes (like match.call
would’ve), and taking care of incrementing the position index the
right amount. Here’s parts of the code:

def self.date_scanner *tags, &block
@@date_scanners << [tags, block]
end

def self.setup_date_scanners
@@date_scanners = []

date_scanner(NLTime::Day, :time, :tz) do |d, t|
  # two timezones were given, like 12:30:00 -0400 (EDT); ignore

rightmost one
d.get_tag(NLTime::Day).time(t.get_tag(NLTime::Time))
end

date_scanner(NLTime::Day, :time) do |d, t|
  d.get_tag(NLTime::Day).time(t.get_tag(NLTime::Time))
end

date_scanner(:time, NLTime::Day) do |t, d|
  d.get_tag(NLTime::Day).time(t.get_tag(NLTime::Time))
end

date_scanner(:month, :num, :time, :year) do |m, a, t, y|
  # May 05 12:00:00 -0000 2005
  day = NLTime::Day.civil(a.word, m.word, y.get_tag(NLTime::Year))
  day.time(t.get_tag(NLTime::Time))
end

date_scanner(:year, :num, :num) do |y, a, b|
  # 2005 05 05
  NLTime::Day.civil(b.word, a.word, y.get_tag(NLTime::Year))
end

date_scanner(:year, :month, :num) do |y, m, a|
  # 2005 May 05
  NLTime::Day.civil(a.word, m.word, y.get_tag(NLTime::Year))
end

date_scanner(:month, :num, :year) do |m, a, y|
  # May 05 2005
  NLTime::Day.civil(a.word, m.word, y.get_tag(NLTime::Year))
end

# ...

end

def self.scan_dates tokens, order=:dm
# TODO:
# order=:dm assume day/month like american format
# order=:md assume month/day like european format

# processed tokens
ptokens = []; k = 0

while k < tokens.size
  found = false

  @@date_scanners.each do |tags, block|
    if s = tokens[-tags.size-k..-1-k]
      # assume success until one of the tags doesn't match
      found = true

      # match tags to tokens
      s.zip(tags).each do |token, tag|
        unless token.has_tag? tag
          # not a match... next scanner, please
          found = false
          break
        end
      end

      if found
        # this scanner matches, have the tokens processed
        if date = block.call(*s)
          token = NLTime::Token.new(date.to_s, :entity, date)
          ptokens.unshift token

          # increment the position by number of tokens processed

by the block
k += tags.size

          # don't try to match any more scanners
          break
        else
          # the block failed, try the next scanner
          found = false
        end

      end
    end
  end

  unless found
    # none of the scanners matched
    ptokens.unshift tokens[-1-k]
    k += 1
  end
end

ptokens

end

The scan_dates operates on an array of NLTime::Tokens, which have
various tags. The tags can be symbols, which basically categorize
words (like “Jan” would have :month tag), or they can be objects (like
we might have tagged 2005 with a NLTime::Year object representing the
year 2005). This should “replace” sequences of tokens that were
matched by a scanner with a new token, tagged with an instance of
NLTime::Day or Time.

I still think you’re not yet there.

Well, my code does what I want it to do… so I’m not sure what you
mean?


#7

On 11.06.2007 22:20, Erwin A. wrote:

tokens with various tags, and I’m matching based on the tags. This is

all the patterns again), and I needed to know if anything had matched
@@date_scanners << [tags, block]

 day = NLTime::Day.civil(a.word, m.word, y.get_tag(NLTime::Year))
 NLTime::Day.civil(a.word, m.word, y.get_tag(NLTime::Year))

def self.scan_dates tokens, order=:dm
@@date_scanners.each do |tags, block|
end
k += tags.size
end

Well, my code does what I want it to do… so I’m not sure what you mean?
I had the impression that your design or implementation still had room
for improvements. From what you wrote I assume you do some kind of
pattern matching. For the fun of it I coded something that solves a
similar problem. Of course this could be changed in all sorts of ways
(i.e. to store the pattern hash as a variable or remove the enum etc.).

Kind regards

robert


#8

Chris C. wrote:

On 6/10/07, Erwin A. removed_email_address@domain.invalid wrote:

puts words.last # output: thre

class Proc
def bcall(*args,&block)
self.call(*args.concat([*block]))
end
end

x_y = lambda {|data,proc| proc.call(data) if data.is_a? String }

x_y.bcall(“hello”) {|data| puts data.succ }

It’s a hack, but it works.

Interesting how an ill-designed syntactic sugar forces people to jump
through the hoops.

Jenda


#9

x = lambda do |*args|
if args.size == 5
# call another block
yield *args.map{|x| x.gsub(/es$/, ‘’) }
end
end

You are using 2 variables with the same name (x) in the same
scope. So, in fact, you are only using 1 variable…

gegroet,
Erik V. - http://www.erikveen.dds.nl/


#10

On 6/12/07, Jenda K. removed_email_address@domain.invalid wrote:

end
Jenda


Posted via http://www.ruby-forum.com/.

Hmm, well, it is a feature that most languages don’t have in the first
place, it exists in later versions of ruby, and I really wouldn’t call
currying “jumping through hoops”, its a pretty common and easy thing
to do…


#11

On 6/10/07, Erwin A. removed_email_address@domain.invalid wrote:

puts words.last # output: thre

class Proc
def bcall(*args,&block)
self.call(*args.concat([*block]))
end
end

x_y = lambda {|data,proc| proc.call(data) if data.is_a? String }

x_y.bcall(“hello”) {|data| puts data.succ }

It’s a hack, but it works.


#12

On 6/13/07, Jenda K. removed_email_address@domain.invalid wrote:

Hmm, well, it is a feature that most languages don’t have in the first

Posted via http://www.ruby-forum.com/.

It is essentially a curry that is called immediately. It expects the
function in question to take a function as the last argument, and
#bcall traps a block, and sets it as that last argument. The
equivalent version that “actually curries” (returns a function) would
be
class Proc
def bcurry(&blk)
proc { |*args| self.call(*args.concat([*block]))}
end
end


#13

Chris C. wrote:

On 6/12/07, Jenda K. removed_email_address@domain.invalid wrote:

end
Jenda


Posted via http://www.ruby-forum.com/.

Hmm, well, it is a feature that most languages don’t have in the first
place, it exists in later versions of ruby, and I really wouldn’t call
currying “jumping through hoops”, its a pretty common and easy thing
to do…

Beg your pardon? Where’s there any currying in your code???
Or does Ruby have its own definition of the term different from the one
everyone else uses?

Jenda