Recently I came upon a groovy oddity. (At least it is perceived by me to be an oddity). Closures in a groovy class do not have access to a private method if that method is defined in the superclass. This seems odd paired against the fact that regular methods in a super class can access private method defined in the super class
Background
Groovy closures have the same scope access to class member variables and methods as a regular groovy method. In other words, closures are bound to variables in the scope they are defined. See the codehaus link for the official documentation:
http://groovy.codehaus.org/Closures
This implies that a closure will play by the rules of Object Orientation in the Java language. However, I found that closures do not have access to private methods that are defined in a super class.
The best way to demonstrate is through a short example from Grails. I have used TDD for the example. Note this is a dummy case with no business purpose. Later on I will offer a more reasonable scenario in the business context where I encountered this scenario.
Showing posts with label groovy. Show all posts
Showing posts with label groovy. Show all posts
Sunday, August 12, 2012
Sunday, May 20, 2012
Grails Dynamic Dropdown
Recently I had a UI requirement where a customer wanted to select values from two separate dropdowns. The value of the first dropdown essentially filtered the values for the second dropdown. Given the financial projects we support are not heavy on UI requirements, I had to do some initial learning and experimentation to yield a good implementation. This blog entry details the how to implement dynamic dropdowns in Grails with ajax and minimal JavaScript.
Example Problem
A contrived for dynamic dropdowns can be described below:
A user would like to select a sports team for a city. The user first selects a value for a dropdown to choose a city. A second dropdown is filtered with the sports teams within that city. An example to clarify:
High Level Design in Grails
Before we get into the details, we can take a step back and describe how we can accomplish a dynamic dropdown in the grails framework.
Domain Objects
The domain objects for this example are quite simple: A City object with a name, and a Team object.
Example Problem
A contrived for dynamic dropdowns can be described below:
A user would like to select a sports team for a city. The user first selects a value for a dropdown to choose a city. A second dropdown is filtered with the sports teams within that city. An example to clarify:
- The user selects Dallas as the city in the first dropdown. The second dropdown now displays values: Mavericks, Cowboys and Rangers.
- The user selects Pittsburgh as the city in the first dropdown. The second dropdown now displays values Steelers, Pirates, and Penguins.
High Level Design in Grails
Before we get into the details, we can take a step back and describe how we can accomplish a dynamic dropdown in the grails framework.
- On a gsp page, create a select dropdown with the list of cities.
- On change of the city dropdown, send an ajax call to the server with a param of the city selected.
- A controller on the server receives the parameter and looks for teams based on the city selected.
- Return a template with a new select dropdown for the teams, providing a model with the filtered list of teams.
Domain Objects
The domain objects for this example are quite simple: A City object with a name, and a Team object.
Monday, January 16, 2012
Groovy DSL - A Simple Example
Domain Specific Languages (DSLs) have become a valuable part of the Groovy idiom. DSLs are used in native Groovy builders, Grails and GORM, and testing frameworks. To a developer, DSLs are consumable and understandable, which makes implementation more fluid as compared to traditional programming. But how is a DSL implemented? How does it work behind the scenes? This article will demonstrate a simple DSL that can get a developer kick started on the basic concepts.
What is a DSL
DSL's are meant to target a particular type of problem. They are short expressive means of programming that fit well in a narrow context. For example with GORM, you can express hibernate mapping with a DSL rather than XML.
A Simple DSL Example in Groovy
The following example offers a simplified view of implementing an internal DSL. Frameworks have much more advanced methods of creating a DSL. However this example does highlight closure delegation and meta object protocol concepts that are essential to understanding the inner workings of a DSL.
What is a DSL
DSL's are meant to target a particular type of problem. They are short expressive means of programming that fit well in a narrow context. For example with GORM, you can express hibernate mapping with a DSL rather than XML.
static mapping = {
table 'person'
columns {
name column:'name'
}
}
Much of the theory for DSLs and the benefits they deliver are well documented. Refer to these sources as a starting point:A Simple DSL Example in Groovy
The following example offers a simplified view of implementing an internal DSL. Frameworks have much more advanced methods of creating a DSL. However this example does highlight closure delegation and meta object protocol concepts that are essential to understanding the inner workings of a DSL.
Wednesday, September 21, 2011
4 Groovy Tips You May Not Know
I have been programming in Groovy full time for almost two years now, and I feel quite comfortable with it. In fact, the language's idioms, shortcuts, and syntax has become part of my daily programming thinking. This is fantastic as it has opened my mind to unique programming elegance and artistry, but unfortunately on the flip-side it has cause me to cringe when looking at traditional Java code. This speaks to the efficiency and conciseness of Groovy.
Recently I decided that even though I am adequetely familiar with the language, I wanted to read Programming In Groovy by Venkat Subramanium. I was pleasantly surprised that I learned some useful tips. Below are some tips that I learned:
1. Variant Looping Mechanisms
In addition to the traditional looping constructs, such as "for (i in iterm)" or range (0..5), Groovy provides alternate ways to loop.
Use the upto function to operate on a range of values:
Use the step function to skip values:
Recently I decided that even though I am adequetely familiar with the language, I wanted to read Programming In Groovy by Venkat Subramanium. I was pleasantly surprised that I learned some useful tips. Below are some tips that I learned:
1. Variant Looping Mechanisms
In addition to the traditional looping constructs, such as "for (i in iterm)" or range (0..5), Groovy provides alternate ways to loop.
Use the upto function to operate on a range of values:
def sum = 0
10.upto(15) {
sum += it
}
println sum
result:
75
Use the step function to skip values:
3.step(20, 3) {
print "$it "
}
result:
3 6 9 12 15 18
Monday, February 21, 2011
5 Handy Groovy Shortcuts
Groovy's main advantage over Java programming is the ability for a developer to implement a solution in fewer lines of code. Groovy's idioms produce concise, short, and clean code. Of course, on first look the code may look strange to a Java developer. But once you learn Groovy's shortcuts you realize the value of consciseness and how less noise produces easier maintenance of code. To me, this is simple: "Since there is less code to look at, there is less code to understand. Learn the shortcuts and idioms and produce less code to do more."
Over the last year, I adopted Groovy as an enterprise implementation language. Here are 5 Groovy programming shortcuts that I found to be very handy. They are listed as tests, in the model of TDD.
Spread Dot Operator
The spread dot operator calls a method on every item in a collection. Upon the invocations, it creates a new list from the return items. So instead of looping through a collection to make a call on each object, use the spread dot operator.
Let's say we want to get the first 3 letters of each username:
Over the last year, I adopted Groovy as an enterprise implementation language. Here are 5 Groovy programming shortcuts that I found to be very handy. They are listed as tests, in the model of TDD.
Spread Dot Operator
The spread dot operator calls a method on every item in a collection. Upon the invocations, it creates a new list from the return items. So instead of looping through a collection to make a call on each object, use the spread dot operator.
Let's say we want to get the first 3 letters of each username:
Sunday, August 15, 2010
3 Common Ways to Use Groovy Closures
What is a Closure?
The concept of closures is an important part of the Groovy language. A closure resembles a function or method in many aspects of programming. For example a closure can take arguments, can have a return value, and can be executed from one or many clients. A closure is essentially a block of code that is defined and executed a later point. A closure can be assigned to a variable or remain anonymous. You can create a closure, assign it to a variable, and then pass it around your program like any other variable. For a more thorough explanation, see Groovy Formal Definition.
Use Groovy Closures
Closures enable the creation of creating concise, clean code when applied correctly. Although all programming problems can be solved without closures, a problem solved using closures will most likely contain less code, avoid repetition, and be more elegant. This article will focus on some simple and common uses of closures. It is directed towards newcomers to Groovy, typically Java developers coming over to the dark side. We will discover how closures can be used for:
The concept of closures is an important part of the Groovy language. A closure resembles a function or method in many aspects of programming. For example a closure can take arguments, can have a return value, and can be executed from one or many clients. A closure is essentially a block of code that is defined and executed a later point. A closure can be assigned to a variable or remain anonymous. You can create a closure, assign it to a variable, and then pass it around your program like any other variable. For a more thorough explanation, see Groovy Formal Definition.
Use Groovy Closures
Closures enable the creation of creating concise, clean code when applied correctly. Although all programming problems can be solved without closures, a problem solved using closures will most likely contain less code, avoid repetition, and be more elegant. This article will focus on some simple and common uses of closures. It is directed towards newcomers to Groovy, typically Java developers coming over to the dark side. We will discover how closures can be used for:
Monday, July 5, 2010
Apply Groovy Curried Closures and Composition to Business Rules
Groovy offers new programming idioms
Groovy is a language that operates on the Java platform and is completely compatible with Java. Many of its features are easily grasped by Java programmers attempting to learn the language. For example, Groovy automatically provides getters and setters for attributes on objects, treats numbers and strings as first class objects, and offers a myriad of programming shortcuts (like << to add to a list or [:] to create a map).
Closures are easy to understand
However, Groovy also contains programming idioms that are not customary in the Java world. These concepts are borrowed from other languages and empower Groovy to be very effective and dynamic in solving business problems. The concept of closures is one example. Closures are a prime feature in Groovy, but do not exist in Java (although Java 7 is scheduled to contain closures). A closure is a defined block of code that can be passed around and executed at a later point. They are commonly used when iterating over collections, and they eliminate much of the code noise that comes with Java iteration. With some effort, closures in their most common application are fairly simple to grasp.
More complex Groovy idioms
Some concepts are more foreign to the conventional Java programmer. Curried closures are an extension to the concept of closure. Currying a closure takes a little more effort to understand and is certainly not a concept common to Java. Groovy also allows for composition of closures, where closures are combined together so that closures operate on other closure results. These techniques are part of the functional programming paradigm which emphasizes the application of functions, rather than methods conducting state changes on data. Haskell, Schema, and Erlang are functional programming languages. Through an example, this article will demonstrate how curried closures and composition can be applied to solving business world problems.
Curry Definition
First let's define what a curried function is. The term curry comes from the mathematician Haskell Curry who developed the concept of partial functions. In functional programming, currying a function means transforming a function into another function by fixing some of its input arguments. Currying is good for mathematical business problems where you need to break the problem down into pieces. Essentially, you have a template mathematical formula and you can assign fixed values to a portion of the formula to create new version of the template. A simple math example follows:
Closures are easy to understand
However, Groovy also contains programming idioms that are not customary in the Java world. These concepts are borrowed from other languages and empower Groovy to be very effective and dynamic in solving business problems. The concept of closures is one example. Closures are a prime feature in Groovy, but do not exist in Java (although Java 7 is scheduled to contain closures). A closure is a defined block of code that can be passed around and executed at a later point. They are commonly used when iterating over collections, and they eliminate much of the code noise that comes with Java iteration. With some effort, closures in their most common application are fairly simple to grasp.
More complex Groovy idioms
Some concepts are more foreign to the conventional Java programmer. Curried closures are an extension to the concept of closure. Currying a closure takes a little more effort to understand and is certainly not a concept common to Java. Groovy also allows for composition of closures, where closures are combined together so that closures operate on other closure results. These techniques are part of the functional programming paradigm which emphasizes the application of functions, rather than methods conducting state changes on data. Haskell, Schema, and Erlang are functional programming languages. Through an example, this article will demonstrate how curried closures and composition can be applied to solving business world problems.
Curry Definition
First let's define what a curried function is. The term curry comes from the mathematician Haskell Curry who developed the concept of partial functions. In functional programming, currying a function means transforming a function into another function by fixing some of its input arguments. Currying is good for mathematical business problems where you need to break the problem down into pieces. Essentially, you have a template mathematical formula and you can assign fixed values to a portion of the formula to create new version of the template. A simple math example follows:
f(x, y) = x / y ---- template f(x, 2) = x / 2 ---- curry with fixed value g(x) = f(x, 2) ---- this is the new curried function g(10) = 10 / 2 = 5
Function Composition Definition
Function composition is the application of one function to the result of the other. Using composition, two or more functions can be combined to produce a more elaborate one. A simple examples follows:
f(x) = x * 2 g(y) = y / 100 f(g(y)) = (y / 100 ) * 2
Investment Business Problem #1
Let's define a business problem that would be interesting to solve with Groovy using curried closures and function composition. One we clearly see the problem definition, we can apply the programming concepts and illustrate the power of its usage.
Let's define a business problem that would be interesting to solve with Groovy using curried closures and function composition. One we clearly see the problem definition, we can apply the programming concepts and illustrate the power of its usage.
Bob wants to invest some money for retirement. He is 55 years old and will retire at 65. He has $5000 total to play with. He has found an investment vehicle that will deliver 8% interest every year for 10 years (compounded once a year). He is not sure whether to invest all of the $5k or only a portion of it. He wants to see the difference.
Used a Curried Closure
Recall that in Groovy, we can create closures which are basically passable functions. Here we can create a closure that encapsulates the investment formula.
Used a Curried Closure
Recall that in Groovy, we can create closures which are basically passable functions. Here we can create a closure that encapsulates the investment formula.
def interestFormula = { r, y, p -> p * Math.pow(1+(r), y) }
In order to reduce the formula to a specific context, we can replace some of the variables with fixed values. Currying will carry out this task. Then we can reuse the closure to test out various principle investments.
package curry.blog
def interestRate = 0.08
def years = 10
def principle1 = 3000
def principle2 = 5000
// create current investment from interest formula with set rates and
// see what differences in savings will pay off
// years down the road
def interestFormula = { r, y, p -> p * Math.pow(1+(r), y) }
def invest = interestFormula.curry(interestRate, years)
println "invest ${principle1} and return: " + invest(principle1)
println "invest ${principle2} and return: " + invest(principle2)
From the example above, the output is:
invest 3000 and return: 6476.7749918183645
invest 5000 and return: 10794.62498636394
Clearly, Bob can see that $5k investment returns almost $4k more than a $3k investment. If he can afford to put all $5k for retirement, it would pay off well.
Investment Business Problem #2
Now we are ten years down the line, and Bob is ready to liquidate his investment. However, he encounters a money savy friend who says he found a firm that will rollover his retirement fund and actually pay him 10% to manage his money. In order to rollover his retirement account, he must pay the existing firm $100. Thus, Bob is considering liquidating his account and paying the government 30% tax to keep his money, or continue to invest by rolling over and gaining 10%, essentially deferring his taxes until later. He wants to evaluate his initial $5k investment against the different paths he can take.
Investment Business Problem #2
Now we are ten years down the line, and Bob is ready to liquidate his investment. However, he encounters a money savy friend who says he found a firm that will rollover his retirement fund and actually pay him 10% to manage his money. In order to rollover his retirement account, he must pay the existing firm $100. Thus, Bob is considering liquidating his account and paying the government 30% tax to keep his money, or continue to invest by rolling over and gaining 10%, essentially deferring his taxes until later. He wants to evaluate his initial $5k investment against the different paths he can take.
Used a Curried Closure and Composition
We can create a closure to represent money after tax is applied. We can then curry it to be specific to the 30% tax bracket.
For a rollover, we can create a simple closure as well. No currying necessary here as there is only on variable.
Bob wants to compare the situation where he liquidates against when he reinvests. A composition closure addresses this nicely. We can create a composition template where you can apply different actions after the investment is liquidated. Curry the composition with the investment closure, then leave the other variables open for the principle and the action. Essentially the resulting closure does the investment result for you, then applies the various actions to the result.
See the entire Groovy script.
Should Bob have the ability refrain from liquidating his investment, he can gain big. His $5k turns into ~$12k. And he can hope that later down the line, he will be in a lower tax bracket and he can keep more of his money. If he withdraws now, he is still in the 30% tax bracket and his investment only comes back with ~$7500.
Conclusion
This business problem can certainly be solved with conventional Java paradigms. However, Groovy is rich in features as well as programming idioms. Groovy gives the developer a chance to solve programs in a concise, elegant fashion. If a business problem has functional and mathematical qualities, consider functional programming to solve the issue.
We can create a closure to represent money after tax is applied. We can then curry it to be specific to the 30% tax bracket.
def taxFormula = { t, p ->; p * (1 - t) }
def applyTax = taxFormula.curry(taxRate)
For a rollover, we can create a simple closure as well. No currying necessary here as there is only on variable.
def doRollover = { z -> (z - rollOverFee) * (1 + rollOverMatching) }
Bob wants to compare the situation where he liquidates against when he reinvests. A composition closure addresses this nicely. We can create a composition template where you can apply different actions after the investment is liquidated. Curry the composition with the investment closure, then leave the other variables open for the principle and the action. Essentially the resulting closure does the investment result for you, then applies the various actions to the result.
def composition = {g, x, f -> f(g(x)) }
def considerPostInvestmentAction = composition.curry(invest)
See the entire Groovy script.
considerPostInvestmentAction calculates the investment, then tells you what the result is after you liquidate it and pay taxes, or roll it over. It can accept a closure and an initial principle. The closure passed in represents the action taken on the liquid value, either tax applied or a rollover. Bob can compare the differences.
package curry.blog
def interestRate = 0.08
def years = 10
def taxRate = 0.30
def principle2 = 5000
def rollOverFee = 100
def rollOverMatching = 0.10
def interestFormula = { r, y, p -> p * Math.pow(1+(r), y) }
def invest = interestFormula.curry(interestRate, years)
// tax formula which tells you how much you will have after taxes
def taxFormula = { t, p -> p * (1 - t) }
// curry it with the tax Rate
def applyTax = taxFormula.curry(taxRate)
// formula for rollover; the amount you will have after rollover
def doRollover = { z -> (z - rollOverFee) * (1 + rollOverMatching) }
// template for functional composition
def composition = {g, x, f -> f(g(x)) }
// use composition and currying to allow you apply a function on top of your
// investment after you sell it. Thus you can decide
// whether to pay taxes and keep the income, or roll over into another account
def considerPostInvestmentAction = composition.curry(invest)
println "initial investment is ${principle2}. After taxes it is: " + considerPostInvestmentAction(principle2, applyTax)
println "initial investment is ${principle2}. After rollover it is: " + considerPostInvestmentAction(principle2, doRollover)
initial investment is 5000. After taxes it is: 7556.237490454758
initial investment is 5000. After rollover it is: 11764.087485000335 Should Bob have the ability refrain from liquidating his investment, he can gain big. His $5k turns into ~$12k. And he can hope that later down the line, he will be in a lower tax bracket and he can keep more of his money. If he withdraws now, he is still in the 30% tax bracket and his investment only comes back with ~$7500.
Conclusion
This business problem can certainly be solved with conventional Java paradigms. However, Groovy is rich in features as well as programming idioms. Groovy gives the developer a chance to solve programs in a concise, elegant fashion. If a business problem has functional and mathematical qualities, consider functional programming to solve the issue.
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