Anonymous functions (functions with no name) are sometimes wrongly called closures. Most languages that have anonymous functions also have closures. An anonymous function is also a closure if it has an environment of its own with at least one bound variable. An anonymous function with no environment of its own is not a closure. A named closure is not anonymous.
"Values" may be numbers or some other type of data, such as letters, or data structures made up of simpler parts. In the rules of a programming language, the first-class values are values that can be given to functions, returned by functions, and bound to a variable name. Functions that take or return other functions are called higher-order functions. Most languages that have functions as first-class values also have higher-order functions and closures.
For example, take a look at the following Scheme function:
In this example, the lambda expression codice_1 is part of the function codice_2. When the function is run, Scheme must make the value of the lambda. It does this by making a closure with the code for the lambda and a reference to the codice_3 variable, which is a free variable inside the lambda. (A "free variable" is a name that is not bound to a value.)
The codice_4 function then runs the closure on each book in the list to pick which books to return. Because the closure itself has a reference to codice_3, the closure can use that value each time codice_4 runs the closure. The function codice_4 itself might be written in a completely separate file.
Here is the same example rewritten in (JavaScript), another popular language with support for closures:
ECMAScript uses the word codice_8 here instead of codice_9, and the codice_10 method in place of the codice_4 function, but otherwise the code does the same thing in the same way.
A function may create a closure and return it. The following example is a function that returns a function.
In Scheme:
In ECMAScript:
The closure environment keeps the bound variables codice_12 and codice_13 after the "enclosing" function (codice_14) returns. In languages without closures, these values would be lost after the enclosing function returns. In languages with closures, a bound variable must be kept in memory as long as any closure has it.
A closure need not be formed using an anonymous function. The Python programming language, for example, has limited support for anonymous functions but does have closures. For example, one way the above ECMAScript example could be implemented in Python is:
In this example, the function named "gradient" makes a closure together with the variables "f" and "dx". The outer enclosing function named "derivative" returns this closure. In this case, an anonymous function would work also.
def derivative(f, dx):
Python must often use named functions instead because its lambda expressions may only contain other "expressions" (code that returns a value) and not "statements" (code that has effects but no value). But in other languages, such as Scheme, all code returns a value; in Scheme, everything is an expression.
Closures have many uses:
In Scheme
Note: Some speakers call any data structure that binds a lexical environment a closure, but the term usually refers specifically to functions.
Agriculture in Pakistan
Pakistan's principal natural resources are arable land and water. About 25% of Pakistan's total land area is under cultivation and is watered by one of the largest irrigation systems in the world. Pakistan irrigates three times more acres than Russia. Agriculture accounts for about 23% of GDP and employs about 44% of the labor force. Pakistan is one of the world's largest producers and suppliers of the following according to the 2005 Food and Agriculture Organization of The United Nations and FAOSTAT given here with ranking:
Pakistan ranks fifth in the Muslim world and twentieth worldwide in farm output. It is the world's fifth largest milk producer.
Agriculture is the back bone of Pakistan's economy.
Gwadar Bay
Gwadar Bay is near the maritime border of Pakistan and Iran. The name is from Persian, "Khalij-e Gavader" or 'Gulf of Gwadar' on Arabian Sea. It is an inlet of the Arabian Sea indenting the sandy Makran coast at the Iran–Pakistan border. It is about 20 miles (32 km) long and 10 miles (16 km) wide. The Dashtiari River flows into it from the northwest, and the Dasht River from the northeast. The town of Gwadar, Pakistan, is on the Arabian Sea coast about 30 miles (48 km) east of Gwadar Bay.
Huntly, Scotland
Huntly () is a town in Aberdeenshire, Scotland, formerly known as Milton of Strathbogie. It has a population 4,460 2004 and is the site of Huntly Castle.
Huntly was the home town of the writer George MacDonald. Some of his novels, especially the Robert Falconer and Alec Forbes of Howglen play partly in Huntly, even if the name of the town is changed. So they give a rather good introduction to the life in Huntly in the 19th century.
The local football team is Huntly F.C..
Clerkhill
Clerkhill is an area of the town of Peterhead in north east Scotland. Surrounded by the areas of Meethill, Dales Park and Coplandhill, it is one of the smaller areas of the town.
Ellon, Aberdeenshire
Ellon is a town in Aberdeenshire, Scotland, approximately 16 miles north of Aberdeen, lying on the River Ythan (pronounced "eye-than") which has one of the few undeveloped river estuaries on the eastern coast of Scotland. It is in the ancient region of Formartine.
Fraserburgh
Fraserburgh (Scots: The Broch) (Scottish Gaelic: A' Bhruaich) is a town in Aberdeenshire, Scotland with a population recorded in the 2001 Census at 12,454. It lies at the northeast corner of Aberdeenshire, around north of Aberdeen, and north of Peterhead. It is the largest shellfish port in Europe and a major white fish port and busy commercial harbour.
Fraserburgh golf club is the fifth oldest club in Scotland and seventh oldest in the world. It has both an 18 hole and a 9 hole course, and a modern clubhouse.
Fraserburgh Football Club is a senior football club that plays in the Highland League. Fraserburgh United FC is a junior football club that plays in the Scottish Junior Football North Premier League (also known as the North Superleague).
Rosehearty
Rosehearty (Gaelic: "Ros Abhartaich"), "Rizarty" in the local dialect, is a village in Scotland. It is on the Moray Firth coast, four miles west of the town Fraserburgh, in the county of Aberdeenshire.
The burgh has about 1,300 people, with about 25 per cent of pensionable age. There are two shops and three hotels in the village.
Stonehaven
Stonehaven ("Steenhive" in the Doric dialect of Scots) and "Cala na Creige" in Gaelic is a town with around fourteen thousand inhabitants (9,577 in the 2001 census) on the northeast coast of Scotland. It is the county town of the historic county of Kincardineshire or The Mearns and the present day county of Aberdeenshire. It grew around an Iron Age fishing village, now the "Auld Toon" ("old town"), and expanded inland from the Seaside. As late as the 16th century, old maps indicate the town was called Stonehyve or Stonehive.
Stonehaven was the birthplace of Robert William Thomson, inventor of the pneumatic tyre and the fountain pen, of journalist James Murdoch and Lord Reith of Stonehaven, first Director-General of the BBC.
Stonehaven was a holiday retreat of the poet, Robert Burns.
The novelist Lewis Grassic Gibbon (James Leslie Mitchell) attended school at what was the old Mackie Academy (now Arduthie Primary). Mackie Academy now serves over 1000 pupils and they study his work.
Famous historical visitors include William Wallace and Mary Queen of Scots.
Turriff
Turriff is a town and parish in Aberdeenshire, Scotland. It is approximately 166 feet above sea level. It is known locally as Turra in the Doric dialect of Scots. The name appears to be Scottish Gaelic in origin, from ""torr"" meaning a mound or round hill, or ""tur"" meaning a tower.