I have some records with similar fields, like this:
-define(COMMON_FIELDS, common1, common2, common3).
-record(item1, a, b, c, ?COMMON_FIELDS).
-record(item2, x, y, z, ?COMMON_FIELDS).
But later I need to write similar code for every record:
Record#item1.common1,
Record#item1.common2,
Record#item1.common3
and:
Record#item2.common1,
Record#item2.common2,
Record#item2.common3
Is there way to write one function for access to same fields in different records?
Is there way to write one function for access to same fields in
different records?
1) Pattern matching in multiple function clauses:
-module(x1).
-export([read/1]).
-define(COMMON_FIELDS, common1, common2, common3).
-record(item1, {x, ?COMMON_FIELDS}). %Note that you defined your records incorrectly.
-record(item2, {y, ?COMMON_FIELDS}).
read(#item1{common1=C1, common2=C2, common3=C3} = _Item) ->
io:format("~p, ~p, ~p~n", [C1, C2, C3]);
read(#item2{common1=C1, common2=C2, common3=C3} = _Item) ->
io:format("~p, ~p, ~p~n", [C1, C2, C3]).
...
25> c(x1).
{ok,x1}
26> rr(x1).
[item1,item2]
27> A = #item1{x=10, common1="hello", common2="world", common3="goodbye"}.
#item1{x = 10,common1 = "hello",common2 = "world",
common3 = "goodbye"}
28> B = #item2{y=20, common1="goodbye", common2="mars", common3="hello"}.
#item2{y = 20,common1 = "goodbye",common2 = "mars",
common3 = "hello"}
29> x1:read(A).
"hello", "world", "goodbye"
ok
30> x1:read(B).
"goodbye", "mars", "hello"
ok
Note the export statement--it's a list of length 1, i.e. the module exports one function. The output shows that the read() function can read records of either type.
2) A case statement:
If for some reason, by stating one function you mean one function clause, you can do this:
read(Item) ->
case Item of
#item1{common1=C1, common2=C2, common3=C3} -> true;
#item2{common1=C1, common2=C2, common3=C3} -> true
end,
io:format("~p, ~p, ~p~n", [C1, C2, C3]).
You can use exprecs parse transform from parse_transe.
-module(parse).
-compile({parse_transform, exprecs}).
-record(item1, {x, common1, common2}).
-record(item2, {y, common1, common2}).
-export_records([item1, item2]).
-export([p/0]).
f() ->
R1 = #item1{x=1, common1=foo1, common2=bar1},
R2 = #item2{y=2, common1=foo2, common2=bar2},
['#get-'(Field, Rec) || Field <- [common1, common2], Rec <- [R1, R2]].
...
1> c(parse).
{ok,parse}
2> parse:f().
[foo1,foo2,bar1,bar2]
It might make sense to factor out the common fields into a single field in each record companies containing a record with all the common data or even a tulple. Then refactor your code to do all common processing to its own function.
You still need to pattern match every top level record to get the common sub record. But somewhere you probably want to do the processing specific to each record kind and there you can already match out the common field.
-record(common, {c1, c2, c3}).
-record(item1, {a, b, c, com}).
...
process_item(#item1{a=A, b=B, c=C, com=Com}) ->
process_abc(A, B, C),
process_common(Com),
...;
process_item(#item2{x=X, y=Y ...
Data structures like this might also be a indication to use the new Map data type instead of records.
Related
I have a list of type (string * (int * int)) list. I want to be able to search through the list, finding the right element by it's string identifier, do a calculation on one of the ints, and then return the full, modified list.
Example:
Given a list
let st = [("a1",(100,10)); ("a2",(50,20)); ("a3",(25,40))]
I'm trying to make a function which gets one of the elements and subtracts number from one of the ints in the tuple.
get ("a2",10) st
//Expected result: st' = [("a1",(100,10)); ("a2",(40,20)); ("a3",(25,40))]
I feel I'm almost there, but am a little stuck with the following function:
let rec get (a,k) st =
match st with
| (a',(n',p'))::rest when a'=a && k<=n' -> (n'-k,p')::rest
| (a',(n',p'))::rest -> (n',p')::get (a,k) rest
| _ -> failwith "Illegal input"
This returns [("a2",(40,20)); ("a3",(25,40))] and is thus missing the first a1 element. Any hints?
Lists are immutable, so if you want to "change one element" you are really creating a new list with one element transformed. The easiest way to do a transformation like this is to use List.map function. I would write something like:
let updateElement key f st =
st |> List.map (fun (k, v) -> if k = key then k, f v else k, v)
updateElement is a helper that takes a key, update function and an input. It returns list where the element with the given key has been transformed using the given function. For example, to increment the first number associated with a2, you can write:
let st = [("a1",(100,10)); ("a2",(50,20)); ("a3",(25,40))]
st |> updateElement "a2" (fun (a, b) -> a + 10, b)
I was looking for a function which would update an element in a list based on the element's data. I couldn't find one in F#5, so wrote one using Tomas' solution:
let updateAt (elemFindFunc: 'a -> bool) (newElem: 'a) (source: 'a list) : 'a list =
source
|> List.map
(fun elem ->
let foundElem = elemFindFunc elem
if foundElem then newElem else elem)
elemFindFunc is the function which consumes an element and returns true if this is the element we want to replace. If this function returns true for multiple elements, then those will be replaced by newElem. Also, if elemFindFunc evaluates to false for all elements, the list will be unaltered.
newElem is the new value you want to replace with. newElem could be replaced by a function like valueFunc: 'a -> 'a if you want to process the element before inserting it.
I am having Data like the below:
Data = [{<<"status">>,<<"success">>},
{<<"META">>,
{struct,[{<<"createdat">>,1406895903.0},
{<<"user_email">>,<<"gopikrishnajonnada#gmail.com">>},
{<<"campaign">>,<<"5IVUPHE42HP1NEYvKb7qSvpX2Cm">>}]}},
{<<"mode">>,1}]
And Now i am having a
FieldList = ['<<"5IVUPHE42HP1NEYvKb7qSvpX2Cm">>']
Now:
I am trying like the below but i am getting empty instead of the value
90> [L || L <- FieldList,proplists:get_value(<<"campaign">>,element(2,proplists:get_value(<<"META">>,Data,{[],[]}))) == L].
[]
so how to get the both values are equal and get the final value.
You can parse the atom as if it were an Erlang term:
atom_to_binary(Atom) ->
L = atom_to_list(Atom),
{ok, Tokens, _} = erl_scan:string(L ++ "."),
{ok, Result} = erl_parse:parse_term(Tokens),
Result.
You can then do
[L ||
L <- FieldList,
proplists:get_value(<<"campaign">>,
element(2,
proplists:get_value(<<"META">>,Data,{[],[]})))
== atom_to_binary(L)
].
You can also do it the other way round, (trying to) convert the binary to an atom using this function:
binary_literal_to_atom(Binary) ->
Literal = lists:flatten(io_lib:format("~p", [Binary])),
try
list_to_existing_atom(Literal)
catch
error:badarg -> undefined
end.
This function will return undefined if the atom is not known yet (s. Erlang: binary_to_atom filling up atom table space security issue for more information on this). This is fine here, since the match can only work if the atom was known before, in this case by being defined in the FieldList variable.
How did you get those values in the first place?
Data = [{<<"status">>,<<"success">>},
{<<"META">>,
{struct,[{<<"createdat">>,1406895903.0},
{<<"user_email">>,<<"gopikrishnajonnada#gmail.com">>},
{<<"campaign">>,<<"5IVUPHE42HP1NEYvKb7qSvpX2Cm">>}]
}
},
{<<"mode">>,1}].
[_,{_,{struct,InData}}|_] = Data.
[X || {<<"campaign">>,X} <- InData].
it gives you the result in the form : [<<"5IVUPHE42HP1NEYvKb7qSvpX2Cm">>]
of course you can use the same kind of code if the tuple {struct,InData} may be in a different place in the Data variable.
-module(wy).
-compile(export_all).
main() ->
Data = [{<<"status">>,<<"success">>},
{<<"META">>,
{struct,[{<<"createdat">>,1406895903.0},
{<<"user_email">>,<<"gopikrishnajonnada#gmail.com">>},
{<<"campaign">>,<<"5IVUPHE42HP1NEYvKb7qSvpX2Cm">>}]
}
},
{<<"mode">>,1}],
Fun = fun({<<"META">>, {struct, InData}}, Acc) ->
Value = proplists:get_value(<<"campaign">>, InData, []),
[Value | Acc];
(_Other, Acc)->
Acc
end,
lists:foldl(Fun, [], Data).
I think you can use this code.
I found myself in the position of needing to increment a value which was deeply nested in a series of erlang records. My first attempts at doing this with list comprehensions were dismal failures. Originally, the list contained a number of records where the target value would be absent because the record that contained it would, at some level, be undefined.
I dealt with that easily enough by using lists:partition to filter out only those entries that actually needed incrementing, but I was still unable to come up with a list comprehension that would do such a simple operation.
The code sample below probably doesn't compile - it is simply to demonstrate what I was trying to accomplish. I put the "case (blah) of undefined" sections to illustrate my original problem:
-record(l3, {key, value}).
-record(l2, {foo, bar, a_thing_of_type_l3}).
-record(l1, {foo, bar, a_thing_of_type_l2}).
increment_values_recursive([], Acc
increment_values_recursive([L1 | L1s], Acc) ->
case L1#l1.a_thing_of_type_l2 of
undefined -> NewRecord = L1;
L2 ->
case L2#l2.a_thing_of_type_l3 of
undefined -> NewRecord = L2;
{Key, Value} ->
NewRecord = L1#l1{l2 = L2#l2{l3 = {Key, Value + 1}}}
end
end,
increment_values_recursive(L1s, [NewRecord | Acc]).
increment_values(L1s) ->
lists:reverse(increment_values_recursive(L1s, [])).
........
NewList = increment_values(OldList).
That was what I started with, but I'd be happy to see a list comprehension that would process this when the list didn't have to check for undefined members. Something like this, really:
increment_values_recursive([], Acc
increment_values_recursive([L1 | L1s], Acc) ->
%I'm VERY SURE that this doesn't actually compile:
#l1{l2 = #l2{l3 = #l3{_Key, Value} = L3} = L2} = L1,
%same here:
NewRecord = L1#l1{l2=L2#l2{l3=L3#l3{value = Value+1}}},
increment_values_recursive(L1s, [NewRecord | Acc]).
increment_values(L1s) ->
lists:reverse(increment_values_recursive(L1s, [])).
AKA:
typedef struct { int key, value; } l3;
typedef struct { int foo, bar; l3 m_l3 } l2;
typedef struct { int foo, bar; l2 m_l2 } l1;
for (int i=0; i<NUM_IN_LIST; i++)
{
objs[i].m_l2.m_l3.value++;
}
You can use a list comprehension and even don't need to filter out records that don't have the nesting.
To avoid readability problems I shortened your record definition.
-record(l3, {key, value}).
-record(l2, {foo, bar, al3}).
-record(l1, {foo, bar, al2}).
Define a helper function to increment the value:
inc_value(#l1{al2=#l2{al3=#l3{value=Value}=L3}=L2}=L1) ->
L1#l1{al2=L2#l2{al3=L3#l3{value=Value+1}}};
inc_value(R) ->
R.
Note the last clause that maps any other stuff that doesn't match the pattern to itself.
Lets define example records to try this out:
1> R=#l1{foo=1, bar=2}.
#l1{foo = 1,bar = 2,al2 = undefined}
This is a record that doesn't have the full nesting defined.
2> R1=#l1{foo=1, bar=2, al2=#l2{foo=3, bar=4, al3=#l3{key=mykey, value=10}}}.
#l1{foo = 1,bar = 2,
al2 = #l2{foo = 3,bar = 4,
al3 = #l3{key = mykey,value = 10}}}
Another one that has the full structure.
Try out the helper function:
4> inc_value(R).
#l1{foo = 1,bar = 2,al2 = undefined}
It leaves alone the not fully nested record.
3> inc_value(R1).
#l1{foo = 1,bar = 2,
al2 = #l2{foo = 3,bar = 4,
al3 = #l3{key = mykey,value = 11}}}
It increments the fully nested record ok.
Now the list comprehension is simple and readable:
5> [ inc_value(X) || X <- [R, R1] ].
[#l1{foo = 1,bar = 2,al2 = undefined},
#l1{foo = 1,bar = 2,
al2 = #l2{foo = 3,bar = 4,
al3 = #l3{key = mykey,value = 11}}}]
This is waaaay messier than it would be in a language with destructive mutation, but it is definitely possible. Here's the dirt:
increment(Records) ->
[L1#l1{l2 = (L1#l1.l2)#l2{l3 = ((L1#l1.l2)#l2.l3)#l3{value = ((L1#l1.l2)#l2.l3)#l3.value + 1}}} || L1 <- Records].
As you can see, this is ugly as hell; furthermore, it's difficult to immediately apprehend what this comprehension is doing. It's straightforward to figure out what's going on, but I'd have a talk with anyone in my shop who wrote something like this. Much better to simply accumulate and reverse - the Erlang compiler and runtime are very good at optimizing this sort of pattern.
It is not as hard as it seems. #Peer Stritzinger gave a good answer, but here is my take, with a clean list comprehension:
-record(l3, {key, value}).
-record(l2, {foo=foo, bar=bar, al3}).
-record(l1, {foo=foo, bar=bar, al2}).
increment(#l1{al2 = Al2}=L1) -> L1#l1{al2 = increment(Al2)};
increment(#l2{al3 = Al3}=L2) -> L2#l2{al3 = increment(Al3)};
increment(#l3{value = V}=L3) -> L3#l3{value = V + 1}.
test() ->
List =
[ #l1{al2=#l2{al3=#l3{key=0, value = 100}}}
, #l1{al2=#l2{al3=#l3{key=1, value = 200}}}
, #l1{al2=#l2{al3=#l3{key=2, value = 300}}}
, #l1{al2=#l2{al3=#l3{key=3, value = 400}}}],
[increment(L) || L <- List].
The best solution is probably to look into the concept of lenses in functional programming. A lens is a functional getter and setter for mutation of records. Done correctly, you can then write higher-order lenses which compose primitive lenses.
The result is that you can construct a mutator for your purpose and then run the mutator through all the records by a comprehension.
It is one of those things I wanna write some day for Erlang but never really got the time to write up :)
I am looking for a way to find tuples in a list in Erlang using a partial tuple, similarly to functors matching in Prolog. For example, I would like to following code to return true:
member({pos, _, _}, [..., {pos, 1, 2}, ...])
This code does not work right away because of the following error:
variable '_' is unbound
Is there a brief way to achieve the same effect?
For simple cases it's better to use already mentioned lists:keymember/3. But if you really need member function you can implement it yourself like this:
member(_, []) ->
false;
member(Pred, [E | List]) ->
case Pred(E) of
true ->
true;
false ->
member(Pred, List)
end.
Example:
>>> member(fun ({pos, _, 2}) -> true; (_) -> false end, [..., {pos, 1, 2}, ...]).
Use lists:keymember/3 instead.
You can do it with a macro using a list comprehension:
-define(member(A,B), length([0 || A <- B])>0).
?member({pos, _, _}, [{width, 17, 42}, {pos, 1, 2}, totally_irrelevant]).
It is not very efficient (it runs through the whole list) but it is the closest I can think to the original syntax.
If you want to actually extract the elements that match you just remove 'length' and add a variable:
-define(filter(A,B), [_E || A =_E <- B]).
You could do it using list comprehension:
Matches = [ Match || {Prefix, _, _} = Match <- ZeList, Prefix == pos].
Another possibility would be to do what match specs do and use the atom '_' instead of a raw _. Then, you could write a function similar to the following:
member(X, List) when is_tuple(X), is_list(List) ->
member2(X, List).
% non-exported helper functions:
member2(_, []) ->
false;
member2(X, [H|T]) when not is_tuple(H); size(X) =/= size(H) ->
member2(X, T);
member2(X, [H|T]) ->
case is_match(tuple_to_list(X), tuple_to_list(H)) of
true -> true;
false -> member2(X, T)
end.
is_match([], []) ->
true;
is_match(['_'|T1], [_|T2]) ->
is_match(T1, T2);
is_match([H|T1], [H|T2]) ->
is_match(T1, T2);
is_match(_, _) ->
false.
Then, your call would now be:
member({pos, '_', '_'}, [..., {pos, 1, 2}, ...])
This wouldn't let you match patterns like {A, A, '_'} (checking where the first two elements are identical), but if you don't need variables this should work.
You could also extend it to use variables using a similar syntax to match specs ('$1', '$2', etc) with a bit more work -- add a third parameter to is_match with the variable bindings you've seen so far, then write function clauses for them similar to the clause for '_'.
Granted, this won't be the fastest method. With the caveat that I haven't actually measured, I expect using the pattern matching in the language using a fun will give much better performance, although it does make the call site a bit more verbose. It's a trade-off you'll have to consider.
May use ets:match:
6> ets:match(T, '$1'). % Matches every object in the table
[[{rufsen,dog,7}],[{brunte,horse,5}],[{ludde,dog,5}]]
7> ets:match(T, {'_',dog,'$1'}).
[[7],[5]]
8> ets:match(T, {'_',cow,'$1'}).
[]
Anyone have a decent example, preferably practical/useful, they could post demonstrating the concept?
(Edit: a small Ocaml FP Koan to start things off)
The Koan of Currying (A koan about food, that is not about food)
A student came to Jacques Garrigue and said, "I do not understand what currying is good for." Jacques replied, "Tell me your favorite meal and your favorite dessert". The puzzled student replied that he liked okonomiyaki and kanten, but while his favorite restaurant served great okonomiyaki, their kanten always gave him a stomach ache the following morning. So Jacques took the student to eat at a restaurant that served okonomiyaki every bit as good as the student's favorite, then took him across town to a shop that made excellent kanten where the student happily applied the remainder of his appetite. The student was sated, but he was not enlightened ... until the next morning when he woke up and his stomach felt fine.
My examples will cover using it for the reuse and encapsulation of code. This is fairly obvious once you look at these and should give you a concrete, simple example that you can think of applying in numerous situations.
We want to do a map over a tree. This function could be curried and applied to each node if it needs more then one argument -- since we'd be applying the one at the node as it's final argument. It doesn't have to be curried, but writing another function (assuming this function is being used in other instances with other variables) would be a waste.
type 'a tree = E of 'a | N of 'a * 'a tree * 'a tree
let rec tree_map f tree = match tree with
| N(x,left,right) -> N(f x, tree_map f left, tree_map f right)
| E(x) -> E(f x)
let sample_tree = N(1,E(3),E(4)
let multiply x y = x * y
let sample_tree2 = tree_map (multiply 3) sample_tree
but this is the same as:
let sample_tree2 = tree_map (fun x -> x * 3) sample_tree
So this simple case isn't convincing. It really is though, and powerful once you use the language more and naturally come across these situations. The other example with some code reuse as currying. A recurrence relation to create prime numbers. Awful lot of similarity in there:
let rec f_recurrence f a seed n =
match n with
| a -> seed
| _ -> let prev = f_recurrence f a seed (n-1) in
prev + (f n prev)
let rowland = f_recurrence gcd 1 7
let cloitre = f_recurrence lcm 1 1
let rowland_prime n = (rowland (n+1)) - (rowland n)
let cloitre_prime n = ((cloitre (n+1))/(cloitre n)) - 1
Ok, now rowland and cloitre are curried functions, since they have free variables, and we can get any index of it's sequence without knowing or worrying about f_recurrence.
While the previous examples answered the question, here are two simpler examples of how Currying can be beneficial for F# programming.
open System.IO
let appendFile (fileName : string) (text : string) =
let file = new StreamWriter(fileName, true)
file.WriteLine(text)
file.Close()
// Call it normally
appendFile #"D:\Log.txt" "Processing Event X..."
// If you curry the function, you don't need to keep specifying the
// log file name.
let curriedAppendFile = appendFile #"D:\Log.txt"
// Adds data to "Log.txt"
curriedAppendFile "Processing Event Y..."
And don't forget you can curry the Printf family of function! In the curried version, notice the distinct lack of a lambda.
// Non curried, Prints 1 2 3
List.iter (fun i -> printf "%d " i) [1 .. 3];;
// Curried, Prints 1 2 3
List.iter (printfn "%d ") [1 .. 3];;
Currying describes the process of transforming a function with multiple arguments into a chain of single-argument functions. Example in C#, for a three-argument function:
Func<T1, Func<T2, Func<T3, T4>>> Curry<T1, T2, T3, T4>(Func<T1, T2, T3, T4> f)
{
return a => b => c => f(a, b, c);
}
void UseACurriedFunction()
{
var curryCompare = Curry<string, string, bool, int>(String.Compare);
var a = "SomeString";
var b = "SOMESTRING";
Console.WriteLine(String.Compare(a, b, true));
Console.WriteLine(curryCompare(a)(b)(true));
//partial application
var compareAWithB = curryCompare(a)(b);
Console.WriteLine(compareAWithB(true));
Console.WriteLine(compareAWithB(false));
}
Now, the boolean argument is probably not the argument you'd most likely want to leave open with a partial application. This is one reason why the order of arguments in F# functions can seem a little odd at first. Let's define a different C# curry function:
Func<T3, Func<T2, Func<T1, T4>>> BackwardsCurry<T1, T2, T3, T4>(Func<T1, T2, T3, T4> f)
{
return a => b => c => f(c, b, a);
}
Now, we can do something a little more useful:
void UseADifferentlyCurriedFunction()
{
var curryCompare = BackwardsCurry<string, string, bool, int>(String.Compare);
var caseSensitiveCompare = curryCompare(false);
var caseInsensitiveCompare = curryCompare(true);
var format = Curry<string, string, string, string>(String.Format)("Results of comparing {0} with {1}:");
var strings = new[] {"Hello", "HELLO", "Greetings", "GREETINGS"};
foreach (var s in strings)
{
var caseSensitiveCompareWithS = caseSensitiveCompare(s);
var caseInsensitiveCompareWithS = caseInsensitiveCompare(s);
var formatWithS = format(s);
foreach (var t in strings)
{
Console.WriteLine(formatWithS(t));
Console.WriteLine(caseSensitiveCompareWithS(t));
Console.WriteLine(caseInsensitiveCompareWithS(t));
}
}
}
Why are these examples in C#? Because in F#, function declarations are curried by default. You don't usually need to curry functions; they're already curried. The major exception to this is framework methods and other overloaded functions, which take a tuple containing their multiple arguments. You therefore might want to curry such functions, and, in fact, I came upon this question when I was looking for a library function that would do this. I suppose it is missing (if indeed it is) because it's pretty trivial to implement:
let curry f a b c = f(a, b, c)
//overload resolution failure: there are two overloads with three arguments.
//let curryCompare = curry String.Compare
//This one might be more useful; it works because there's only one 3-argument overload
let backCurry f a b c = f(c, b, a)
let intParse = backCurry Int32.Parse
let intParseCurrentCultureAnyStyle = intParse CultureInfo.CurrentCulture NumberStyles.Any
let myInt = intParseCurrentCultureAnyStyle "23"
let myOtherInt = intParseCurrentCultureAnyStyle "42"
To get around the failure with String.Compare, since as far as I can tell there's no way to specify which 3-argument overload to pick, you can use a non-general solution:
let curryCompare s1 s2 (b:bool) = String.Compare(s1, s2, b)
let backwardsCurryCompare (b:bool) s1 s2 = String.Compare(s1, s2, b)
I won't go into detail about the uses of partial function application in F# because the other answers have covered that already.
It's a fairly simple process. Take a function, bind one of its arguments and return a new function. For example:
let concatStrings left right = left + right
let makeCommandPrompt= appendString "c:\> "
Now by currying the simple concatStrings function, you can easily add a DOS style command prompt to the front of any string! Really useful!
Okay, not really. A more useful case I find is when I want to have a make a function that returns me data in a stream like manner.
let readDWORD array i = array[i] | array[i + 1] << 8 | array[i + 2] << 16 |
array[i + 3] << 24 //I've actually used this function in Python.
The convenient part about it is that rather than creating an entire class for this sort of thing, calling the constructor, calling obj.readDWORD(), you just have a function that can't be mutated out from under you.
You know you can map a function over a list? For example, mapping a function to add one to each element of a list:
> List.map ((+) 1) [1; 2; 3];;
val it : int list = [2; 3; 4]
This is actually already using currying because the (+) operator was used to create a function to add one to its argument but you can squeeze a little more out of this example by altering it to map the same function of a list of lists:
> List.map (List.map ((+) 1)) [[1; 2]; [3]];;
val it : int list = [[2; 3]; [4]]
Without currying you could not partially apply these functions and would have to write something like this instead:
> List.map((fun xs -> List.map((fun n -> n + 1), xs)), [[1; 2]; [3]]);;
val it : int list = [[2; 3]; [4]]
I gave a good example of simulating currying in C# on my blog. The gist is that you can create a function that is closed over a parameter (in my example create a function for calculating the sales tax closed over the value of a given municipality)out of an existing multi-parameter function.
What is appealing here is instead of having to make a separate function specifically for calculating sales tax in Cook County, you can create (and reuse) the function dynamically at runtime.