Related
I am currently trying to create a (sort of) typesafe xml like syntax embedded into Haskell. In the end I am hoping to achieve something like this:
tree = group [arg1 "str", arg2 42]
[item [foo, bar] []
,item [foo, bar] []
]
where group and item are of kind Node :: [Arg t] -> [Node c] -> Node t. If this doesn't make any sense it is most probably because I have no idea what I am doing :)
My question now is how to make the type system prevent me from giving 'wrong' arguments to a Node. Eg Nodes of type Group only may have arguments of type Arg1 and Arg2 but Items may have arguments of type Foo and Bar.
I guess the bottom line question is: how do i restrict the types in a heterogenous list?
Example of the (user) syntax i am trying to achieve:
group .: arg1 "str" .: arg2 42
item .: foo .: bar
item .: foo .: bar
where (.:) is a function that sets the parameter in the node. This would represent a group with some parameters containing two items.
Additionally there would be some (pseudo) definition like:
data Node = Node PossibleArguments PossibleChildNodes
type Group = Node [Arg1, Arg2] [Item]
type Item = Node [Foo, Bar] []
I am searching for a way to catch usage errors by the typechecker.
What you have doesn't sound to me like you need a heterogeneous list. Maybe you're looking for something like this?
data Foo = Foo Int
data Bar = Bar Int
data Arg = StringArg String | IntArg Int | DoubleArg Double
data Tree = Group Arg Arg [Item]
data Item = Item Foo Bar
example :: Tree
example = Group (StringArg "str") (IntArg 42)
[Item (Foo 1) (Bar 2), Item (Foo 12) (Bar 36)]
Note that we could even create a list of Args of different "sub-types". For example, [StringArg "hello", IntArg 3, DoubleArg 12.0]. It would still be a homogeneous list, though.
===== EDIT =====
There are a few ways you could handle the "default argument" situation. Suppose the Bar argument in an item is optional. My first thought is that while it may be optional for the user to specify it, when I store the data I want to include the default argument. That way,
determining a default is separated from the code that actually does something with it. So,
if the user specifies a Foo of 3, but doesn't supply a Bar, and the default is Bar 77, then I create my item as:
Item (Foo 3) (Bar 77)
This has the advantage that functions that operate on this object don't need to worry about defaults; both parameters will always be present as far as they are concerned.
However, if you really want to omit the default arguments in your data structure, you could do somthing like this:
data Bar = Bar Int | DefaultBar
example = Group (StringArg "str") (IntArg 42)
[Item (Foo 1) (Bar 2), Item (Foo 12) DefaultBar]
Or even:
data Item = Item Foo Bar | ItemWithDefaultBar Foo
===== Edit #2 =====
So perhaps you could use something like this:
data ComplicatedItem = ComplicatedItem
{
location :: (Double, Double),
size :: Int,
rotation :: Double,
. . . and so on . . .
}
defaultComplicatedItem = ComplicatedItem { location = (0.0,0.0), size = 1, rotation = 0.0), ... }
To create a ComplicatedItem, the user only has to specify the non-default parameters:
myComplicatedItem = defaultComplicatedItem { size=3 }
If you add new paramters to the ComplicatedItem type, you need to update defaultComplicatedItem, but the definition for myComplicatedItem doesn't change.
You could also override the show function so that it omits the default parameters when printing.
Based on the ensuing discussion, it sounds like what you want is to create a DSL (Domain-Specific Language) to represent XML.
One option is to embed your DSL in Haskell so it can appear in Haskell source code. In general, you can do this by defining the types you need, and providing a set of functions to work with those types. It sounds like this is what you're hoping to do. However, as an embedded DSL, it will be subject to some constraints, and this is the problem you're encountering. Perhaps there is a clever trick to do what you want, maybe something involving type functions, but I can't think of anything at present. If you want to keep trying, maybe add the tags dsl and gadt to your question, catch the attention of people who know more about this stuff than I do. Alternatively, you might be able to use something like Template Haskell or Scrap Your Boilerplate to allow your users to omit some information, which would them be "filled in" before Haskell "sees" it.
Another option is to have an external DSL, which you parse using Haskell. You could define a DSL, but maybe it would be easier to just use XML directly with a suitable DTD. There are Haskell libraries for parsing XML, of course.
I'm looking to call functions dynamically based on the contents found in an association list.
Here is an example in semi-pseudo-code. listOfFunctions would be passed to callFunctions.
listOfFunctions = [('function one', 'value one')
, ('function two', 'value two')
, ('function three', 'value three')]
callFunctions x = loop through functions
if entry found
then call function with value
else do nothing
The crux of the question is not looping through the list, rather, it's how to call a function once I have it's name?
Consider this use case for further clarification. You open the command prompt and are presented with the following menu.
1: Write new vHost file
2: Exit
You write the new vHost file and are not presented with a new menu
1: Enter new directive
2: Write file
3: Exit
You enter some new directives for the vHost and are now ready to write the file.
The program isn't going to blindly write each and every directive it can, rather, it will only write the ones that you supplied. This is where the association list comes in. Writing a giant if/then/else or case statement is madness. It would be much more elegant to loop through the list, look for which directives were added and call the functions to write them accordingly.
Hence, loop, find a function name, call said function with supplied value.
Thanks to anyone who can help out with this.
Edit:
Here is the solution that I've come up with (constructive critiques are always welcome).
I exported the functions which write the directives in an association list as every answer provided said that just including the function is the way to go.
funcMap = [("writeServerName", writeServerName)
,("writeServeralias", writeServerAlias)
,("writeDocRoot", writeDocRoot)
,("writeLogLevel", writeErrorLog)
,("writeErrorPipe", writeErrorPipe)
,("writeVhostOpen", writeVhostOpen)]
In the file which actually writes the hosts, that file is imported.
I have an association list called hostInfo to simulate some dummy value that would be gathered from an end-user and a function called runFunction which uses the technique supplied by edalorzo to filter through both the lists. By matching on the keys of both lists I ensure that the right function is called with the right value.
import Vhost.Directive
hostInfo = [("writeVhostOpen", "localhost:80")
,("writeServerName", "norics.com")]
runFunctions = [f val | (mapKey, f) <- funcMap, (key, val) <- hostInfo, mapKey == key]
You can simply include the function in the list directly; functions are values, so you can reference them by name in a list. Once you've got them out of the list, applying them is just as simple as func value. There's no need to involve their names at all.
Since I am farily new to Haskell I will risk that you consider my suggestion very naive, but anyways here it goes:
let funcs = [("sum", (+3),1),("product", (*3),2),("square", (^2),4)]
[f x | (name, f, x) <- funcs, name == "sum"]
I think it satisfies the requirements of the question, but perhaps what you intend is more sofisticated than what I can see with my yet limitted knowledge of Haskell.
It might be a bit of an overkill (I agree with ehird's reasoning) but you can evaluate a string with Haskell code by using the eval function in System.Eval.Haskell.
EDIT
As pointed out in the comments, hint is a better option for evaluating strings with Haskell expressions. Quoting the page:
This library defines an Interpreter monad. It allows to load Haskell modules, browse them, type-check and evaluate strings with Haskell expressions and even coerce them into values. The library is thread-safe and type-safe (even the coercion of expressions to values). It is, esentially, a huge subset of the GHC API wrapped in a simpler API. Works with GHC 6.10.x and 6.8.x
First we define our list of functions. This could be built using more machinery, but for the sake of example I just make one explicit list:
listOfFunctions :: [(Int, IO ())]
listOfFunctions = [(0, print "HI") -- notice the anonymous function
,(1, someNamedFunction) -- and something more traditional here
]
someNamedFunction = getChar >>= \x -> print x >> print x
Then we can select from this list however we want and execute the function:
executeFunctionWithVal :: Int -> IO ()
executeFunctionWithVal v = fromMaybe (return ()) (lookup v listOfFunctions)
and it works (if you import Data.Maybe):
Ok, modules loaded: Main.
> executeFunctionWithVal 0
"HI"
> executeFunctionWithVal 01
a'a'
'a'
Don't store the functions as strings, or rather, try storing the actual functions and then tagging them with a string. That way you can just call the function directly. Functions are first class values, so you can call the function using whatever name you assign it to.
I know Haskell isn't OO so it isn't strictly a 'member variable'.
data Foo = Foo {
bar :: Int,
moo :: Int,
meh :: Int,
yup :: Int
}
modifyBar (Foo b m me y) = (Foo b' m me y)
where b' = 2
This is how my code looks at the moment. The problem is I am now making data types with 16 or more members. When I need to modify a single member it results in very verbose code. Is there a way around this?
modifyBar foo = foo { bar = 2 }
This syntax will copy foo, and then modify the bar field of that copy to 2. This could be naturally extended to more fields, so you don't need to write that modifyBar function at all.
(See http://book.realworldhaskell.org/read/code-case-study-parsing-a-binary-data-format.html#id625467)
Haskell's "record syntax" that #KennyTM shows is the built-in way to do this, though keep in mind that it's still just a way of constructing a new value based on the old one.
There are some annoying limitations to record syntax, though, particularly that the form used to "modify" a single item in a record aren't first-class entities in the language, so you can't abstract over them and pass them around the way you'd do with a regular function.
An alternative is using a library such as fclabels which provides similar functionality, using Template Haskell to auto-generate accessor functions instead of built-in syntax. The result is often much nicer, with the downside that you now have a dependency on TH....
It seems as though I'm having problems accessing Integer.parseInt in comp. I can access it normally like this:
user=> (Integer/parseInt "123")
123
But if I put it in comp, I get an error:
user=> (def vect-to-int (comp Integer/parseInt (partial apply str)))
java.lang.Exception: Unable to find static field: parseInt in class java.lang.Integer (NO_SOURCE_FILE:3)
It sounds to me like it's trying to find a field on Integer when it should be looking for a method. How can I use Integer.parseInt like this? And is there a better way to convert a vector of chars into an int?
Clojure functions are java methods but java methods are not clojure functions For instance Clojure functions have meta-data and such. If you want to use a java method where a Clojure function is called for then you have two choices in wrapping it up, memfn and fun or #( ) memfn is an obsolete function that wrapped up a java method in a clojure function (its good to know it exists even if its not used often). The generally accepted way to wrap up java methods is:
#(. instance method arg1 argN)
or for static methods
#(Class/MethodName arg1 argN)
(def vec-to-int (comp #(Integer/parseInt %) (partial apply str)))
Or
(def vec-to-int #(Integer/parseInt (apply str %)))
partial and comp usually aren't as succinct in Clojure as using #() to make an anonymous fn. Personally I'd write it this way:
(defn vec-to-int [x] (Integer/parseInt (apply str x)))
You can do this with plain def, but what do you gain using def instead of defn, really? Using def obscures the fact that you're defining a function. defn also sets up additional metadata for you (arglists) that def doesn't.
In any case, this is a more general way to do what you're doing:
(def vec-to-int #(bigint (apply str %)))
And still more general:
(def vec-to-int #(read-string (apply str %)))
Depends whether you want to be able to handle things other than Integers.
I recently had the necessity of rewriting a javascript function in javascript, dynamically. The ease with which I did it, and how fun it was, astounded me.
Over here I've got some HTML:
<div id="excelExport1234"
onclick="if(somestuff) location.href='http://server/excelExport.aspx?id=56789&something=else'; else alert('not important');"
>Click here to export to excel</div>
And I couldn't change the outputted HTML, but I needed to add an extra parameter to that link. I started thinking about it, and realized I could just do this:
excelExport = $('excelExport1234');
if (needParam)
eval('excelExport.onclick = ' + excelExport.onclick.toString().replace("excelReport.aspx?id", "excelReport.aspx?extraParam=true&id") + ';');
else
eval('excelExport.onclick = ' + excelExport.onclick.toString().replace("extraParam=true&", "") + ';');
And it worked like a champ! excelExport.onclick returns a function object which I convert to a string, and do some string manip on. Since it's now in the form of "function() { ... }", I just go back and assign it to the onclick event of the dom object. It's a little ugly having to use eval, but AFAIK there isn't a javascript function constructor that can take a string of code and turn it into an object nicely.
Anyway, my point isn't that I'm super clever (I'm not), my point is that this is cool. And I know javascript isn't the only language that can do this. I've heard that lisp has had macros for years for this exact purpose. Except to really grok macros you need to really grok lisp, and I don't grok it, I just 'kind of get it'.
So my question is: In what other languages can you (easily) dynamically rewrite functions, and can you show me a simple example? I want to see where else you can do this, and how it's done!
(also, I have no idea what to tag this as, so I took random guesses)
LISP is the ultimate language at this. LISP functions are actual LISP lists, meaning you can manipulate LISP source code as if it were any other data structure.
Here's a very trivial example of how it works:
(define hi
(lambda () (display "Hello World\n")))
;; Displays Hello World
(hi)
(set! hi
(lambda () (display "Hola World\n")))
;; Displays Hola World
(hi)
This, however, is possible in any language where functions are first-class objects. One of the most interesting showcases of the power of this syntax for LISP is in its macro system. I really don't feel I could do the topic justice, so read these links if you're interested:
http://en.wikipedia.org/wiki/Macro_(computer_science)#Lisp_macros
http://cl-cookbook.sourceforge.net/macros.html
I guess it depends on what exactly you define as "easily dynamic rewriting". For example in .Net you have the Func type and lambdas which allows you to define functions as variables or as temporary anonymous functions eg.
int[] numbers = {1, 2, 3, 4, 5};
Func<int[], int> somefunc;
if (someCondition)
{
somefunc = (is => is.Sum());
} else {
somefunc = (is => is.Count());
}
Console.WriteLine(somefunc(numbers).ToString());
The above is a very contrived example of either counting the items in an array of integers or summing then using dynamically created functions subject to some arbitrary condition.
Note - Please don't point out that these things can be easily accomplished without lambdas (which they obviously can) I was simply trying to write a very simple example to demonstrate the concept in C#
Self-modifying code is also called degenerate code. This is generally considered a bad thing, and it used to be a goal of high-level languages to prevent it from being written easily.
This is from the wikipedia entry:
Self-modifying code is seen by some as a bad practice which makes code harder to read and maintain. There are however ways in which self modification is nevertheless deemed acceptable, such as when sub routine pointers are dynamically altered - even though the effect is almost identical to direct modification.
I think that it is the case in most of dynamic languages. Here is an example in Python
def f(x):
print x
def new_function(x): print "hello", x
f("world")
f = new_function
f("world")
The output is
world
hello world
I think that such technique should be used carefully
Scheme allows you to do that.
(define (salute-english name) (display "Hello ") (display name))
(define (salute-french nom) (display "Salut ") (display nom))
Now you redefine a fonction by assigning the salute variable to the right function, either salute-english or salute-french, like this:
(define salute salute-english)
(define (redefined-the-salute-function language)
(if (eq? language 'french)
(set! salute salute-french)
(set! salute salute-english)))
More generaly functional programming language allows you to do that or as functions are first class value. Functions can be manipulated, passed around, sometimes assigned to variables and so on. The list then include: Lisp, Scheme, Dylan, OCaml and SML. Some languages having first class functions includes Python, Ruby, Smalltalk and i think Perl.
Note that when you have an interactive language where you can interactively type your program, the redefinition of functions/methods must be possible: the REPL has to be able to do that, just in case you happen to retype the definition of an already defined functions.
I used to do this all the time in TCL, it was a breeze and worked wonderfully. I could investigate somethings interface over the network and then create a custom-made interface on the fly to access and control things. For example, you could make a custom SNMP interface from a generic SNMP library.
I haven't used it, but C# has some built-in support for generating it's own byte-code, which is fairly impressive.
I've done this sort of thing in C as well, but there it is non-portable and almost never worth the hassle. It is a technique used sometimes for "self-optimizing" code to generate the appropriate C function to optimally process a given data set.
You could do it in C++, but it wouldn't be easy, safe, or recommended.
Generate the text of the source code
invoke the compiler (fork & exec) to build a dynamic library. In gcc, you can pass the source code you want to compile on standard input, it doesn't have to be in a file.
Load the library (LoadLibrary() on windows, dlopen() on linux)
get a function pointer to whatever function you want (GetProcAddress() on windows, dlsym() on linux)
If you want to replace an existing function, if it's a virtual function you could modify the v-table to point to the new function (that part especially is a horrible idea fraught with peril). The location of the v-table or the format of it isn't part of the C++ standard, but all the toolchains I've used have been consistent within themselves, so once you figure out how they do it, it probably won't break.
Easy enough in Perl.
*some_func = sub($) {
my $arg = shift;
print $arg, "\n";
};
some_func('foo');
Re Sam Saffron's request:
*hello_world = sub() {
print "oops";
};
hello_world();
*hello_world = sub() {
print "hello world";
};
hello_world();
In PLSQL:
create or replace procedure test
as
begin
execute immediate '
create or replace procedure test2
as
begin
null;
end;
';
end;
/
Here's something else in Python (in addition to luc's answer), which I am not recommending, but just to show it - there is exec, which can execute a string which you could build to be whatever code...
I/O shown here is from a Python 2.5.2 interpreter session. Just some simple examples of constructing strings to execute from substrings (>>> is the interpreter prompt)...
>>> def_string = 'def my_func'
>>> param_string_1 = '():'
>>> param_string_2 = '(x):'
>>> do_string_1 = ' print "Do whatever."'
>>> do_string_2 = ' print "Do something with", x'
>>> do_string_3 = ' print "Do whatever else."'
>>> do_string_4 = ' print "Do something else with", x'
>>> def_1 = '\n'.join([def_string+param_string_1, do_string_1, do_string_3])
>>> print def_1
def my_func():
print "Do whatever."
print "Do whatever else."
>>> exec def_1
>>> my_func()
Do whatever.
Do whatever else.
>>> def_2 = '\n'.join([def_string+param_string_2, do_string_2, do_string_4])
>>> print def_2
def my_func(x):
print "Do something with", x
print "Do something else with", x
>>> exec def_2
>>> my_func('Tom Ritter')
Do something with Tom Ritter
Do something else with Tom Ritter
>>>
Trivial in Ruby:
def hello_world; puts "oops"; end
hello_world
# oops
def hello_world; puts "hello world"; end
hello_world
# hello world
Of course that example is boring:
require "benchmark"
# why oh _why
class Object
def metaclass; class << self; self; end; end
def meta_eval &blk; metaclass.instance_eval &blk; end
end
class Turtle
end
def make_it_move(klass)
klass.send(:define_method, :move) { |distance|
puts "moving #{distance} meters"
sleep(0.1 * distance)
}
end
make_it_move(Turtle)
turtle = Turtle.new
turtle.move(1)
# moving 1 meters
def profile(instance, method)
instance.meta_eval do
m = instance_method(method)
define_method method do |*a|
puts "Benchmarking #{instance.class} #{method}"
puts Benchmark.measure {
m.bind(instance).call(*a)
}
end
end
end
profile(turtle, :move)
turtle.move(10)
# Benchmarking Turtle move
# moving 10 meters
# 0.000000 0.000000 0.000000 ( 1.000994)
Turtle.new.move(3)
# moving 3 meters
The code above:
Defines a blank class
Adds a method to it
Grabs an instance
Intercepts that method on that instance only
Changing what a function does is supported in a lot of languages, and it's not as complicated as you might think. In functional languages, functions are values, and function names are symbols that are bound to them like any variable. If the language allows you to reassign the symbol to a different function, this is trivial.
I think the more interesting features are the ability to get the source code for a function (toString above) and to create a new function from a string (eval in this case).