I want to refactor some C code using Clair and Rascal.
I search for a function with a certain name. If I find such a function, I want to replace it with another function. I need to choose between four functions. The function to choose depends on the argument of the function found. I need to match the root element of an expression.
I can match using the visit pattern. I tried
visit(body) {
case \functionCall(func, args): {
if ("myName" == func.name.\value) {
visit(args[0]) {
case \equals(_, _): println("Equals");
case \notEquals(_, _): println("Not equals");
}
}
}
}
This does not guarantee I match the root element.
In (A != B) == C I only want to match the ==
How can I match only the root element?
You can nest patterns arbitrarily and use this to match a function call including the name of your function and the shape of the first argument you want to match.
For example:
case functionCall(someId("my name", [e:\equals(_, _), *_]) => newFunctionCall(e)
case functionCall(someId("my name", [e:\notEquals(_, _), *_]) => newFunctionCall(e)
Note the list pattern [..] which matches argument lists of arbitrary length here as long as the first argument is an equals or nonEquals expression.
So you could repeat a top-level case for each case of the first parameter, like above, or nest a switch and use "insert", like below:
case functionCall(someId("my name", [arg, *_]) :
switch(arg) {
case equals(_, _) : insert newFunctionCall(arg);
...
}
I prefer the first case because it's more declarative. Rascal should factor the common stuff for efficiency sake internally. The fact that both patterns are very similar is not a code smell in Rascal IMHO, because that's the whole point of this code, you want to treat two similar patterns slightly differently and the first example documents that explicitly without nesting control flow. In other words: It's clearer to nest a pattern than to nest control flow 😁
I use this field, because space is limited in the comment field.
I like this feature of nested patterns, but I do not get the simple case to match. I tried the following patterns:
visit(body) {
case \functionCall("methodA", [arg, *_]): println("match");
case \functionCall(SomeId("methodA", [arg, *_])): println("match");
case \functionCall(SomeId("methodA"), [arg, *_]): println("match");
case \functionCall(IdExpresssion("methodA", [arg, *_])): println("match");
case \functionCall(IdExpresssion("methodA"), [arg, *_]): println("match");
case \functionCall(name("methodA", [arg, *_])): println("match");
case \functionCall(name("methodA"), [arg, *_]): println("match");
}
iprint(body) gives the following results:
compoundStatement(
[
compoundStatement(
[
expressionStatement(
.... some expressions and declarations
for(
... for statement
compoundStatement(
[
... inside for loop
expressionStatement(
functionCall(
idExpression(
name(
"methodA",
src=|project://Instrumentation/example.c|(11195,10)),
src=|project://Instrumentation/example.c|(11195,10),
decl=|cpp+problem://Attempt%20to%20use%20symbol%20failed:%20methodA|,
typ=problemType("Failure to determine type of expression")),
[equals(
idExpression(
name(
"beforeTest",
src=|project://Instrumentation/example.c|(11207,20)),
src=|project://Instrumentation/example.c|(11207,20),
decl=|cpp+variable:///test1()/beforeTest|,
typ=problemType("Type depends on an unresolved name")),
idExpression(
name(
"afterTest",
src=|project://Instrumentation/example.c|(11231,19)),
src=|project://Instrumentation/example.c|(11231,19),
decl=|cpp+variable:///test1()/afterTest|,
typ=problemType("Type depends on an unresolved name")),
src=|project://Instrumentation/example.c|(11207,43),
typ=problemType("Type depends on an unresolved name"))],
src=|project://Instrumentation/example.c|(11195,57),
typ=problemBinding()),
src=|project://Instrumentation/example.c|(11195,58)),
expressionStatement(
... more expressions and declaratios in for loop
],
src=|project://Instrumentation/example.c|(10148,1349))ok
How can I make a pattern that does match MethodA?
Related
like similar question: Convert match statement to partial function when foreach is used. Now similarly, IntelliJ asks me to improve my code. The difference is, that I use values for the matching:
val matchMe = "Foo"
keys.foreach(key =>
key match {
case `matchMe` => somethingSpecial()
case _ => somethingNormal(key, calcWith(key))
})
Refactoring this to a anonymous pattern-matching function would look something like:
keys.foreach {
case `matchMe` => somethingSpecial(_)
case _ => somethingNormal(_, calcWith(_)) //this doesn't work
}
Note that in the second case, I cannot use _ since I need it twice. Is there some way to use an anonymous pattern-matching function here?
You can't use the wildcard _ here, its purpose is to indicate you don't care about the value you're matching against.
You can use a named parameter :
keys.foreach {
case `matchMe` => somethingSpecial(matchMe)
case nonSpecialKey => somethingNormal(nonSpecialKey, calcWith(nonSpecialKey))
}
Without any restrictions placed on it, it will match any value. Do note that the order of cases is important, as case x => ... match anything and will essentially shortcut other case statements.
As an aside, I don't think your somethingSpecial(_) does what you want/expect it to. It's only a short version of x => somethingSpecial(x), not somethingSpecial(matchMe).
I would like to match on tuple pattern, but I can not find any solution how to match using comparison operators. My code is:
myTuple match {
case (-1,-1,true) => ...
case (_>=0,-1,_) => ...
}
This gives give compile time error.
I also tried to use if guard, but as I see it can not be applied this way:
case (_ if _>=0,-1,_) => ...
Is my approach correct or should I solve this on an different way?
Thanks
Zoltan
The syntax is wrong, you should use guard as follows:
myTuple match {
case (-1,-1,true) => ...
case (x,-1,_) if x >= 0 => ...
case _ => ... // default
}
There are a lot of good introduction to scala pattern matching on the web. Here is the first detailed one, I've found on google: Playing with Scala's pattern matching
I found myself writing something like this quite often:
a match {
case `b` => // do stuff
case _ => // do nothing
}
Is there a shorter way to check if some value matches a pattern? I mean, in this case I could just write if (a == b) // do stuff, but what if the pattern is more complex? Like when matching against a list or any pattern of arbitrary complexity. I'd like to be able to write something like this:
if (a matches b) // do stuff
I'm relatively new to Scala, so please pardon, if I'm missing something big :)
This is exactly why I wrote these functions, which are apparently impressively obscure since nobody has mentioned them.
scala> import PartialFunction._
import PartialFunction._
scala> cond("abc") { case "def" => true }
res0: Boolean = false
scala> condOpt("abc") { case x if x.length == 3 => x + x }
res1: Option[java.lang.String] = Some(abcabc)
scala> condOpt("abc") { case x if x.length == 4 => x + x }
res2: Option[java.lang.String] = None
The match operator in Scala is most powerful when used in functional style. This means, rather than "doing something" in the case statements, you would return a useful value. Here is an example for an imperative style:
var value:Int = 23
val command:String = ... // we get this from somewhere
command match {
case "duplicate" => value = value * 2
case "negate" => value = -value
case "increment" => value = value + 1
// etc.
case _ => // do nothing
}
println("Result: " + value)
It is very understandable that the "do nothing" above hurts a little, because it seems superflous. However, this is due to the fact that the above is written in imperative style. While constructs like these may sometimes be necessary, in many cases you can refactor your code to functional style:
val value:Int = 23
val command:String = ... // we get this from somewhere
val result:Int = command match {
case "duplicate" => value * 2
case "negate" => -value
case "increment" => value + 1
// etc.
case _ => value
}
println("Result: " + result)
In this case, you use the whole match statement as a value that you can, for example, assign to a variable. And it is also much more obvious that the match statement must return a value in any case; if the last case would be missing, the compiler could not just make something up.
It is a question of taste, but some developers consider this style to be more transparent and easier to handle in more real-world examples. I would bet that the inventors of the Scala programming language had a more functional use in mind for match, and indeed the if statement makes more sense if you only need to decide whether or not a certain action needs to be taken. (On the other hand, you can also use if in the functional way, because it also has a return value...)
This might help:
class Matches(m: Any) {
def matches[R](f: PartialFunction[Any, R]) { if (f.isDefinedAt(m)) f(m) }
}
implicit def any2matches(m: Any) = new Matches(m)
scala> 'c' matches { case x: Int => println("Int") }
scala> 2 matches { case x: Int => println("Int") }
Int
Now, some explanation on the general nature of the problem.
Where may a match happen?
There are three places where pattern matching might happen: val, case and for. The rules for them are:
// throws an exception if it fails
val pattern = value
// filters for pattern, but pattern cannot be "identifier: Type",
// though that can be replaced by "id1 # (id2: Type)" for the same effect
for (pattern <- object providing map/flatMap/filter/withFilter/foreach) ...
// throws an exception if none of the cases match
value match { case ... => ... }
There is, however, another situation where case might appear, which is function and partial function literals. For example:
val f: Any => Unit = { case i: Int => println(i) }
val pf: PartialFunction[Any, Unit] = { case i: Int => println(i) }
Both functions and partial functions will throw an exception if called with an argument that doesn't match any of the case statements. However, partial functions also provide a method called isDefinedAt which can test whether a match can be made or not, as well as a method called lift, which will turn a PartialFunction[T, R] into a Function[T, Option[R]], which means non-matching values will result in None instead of throwing an exception.
What is a match?
A match is a combination of many different tests:
// assign anything to x
case x
// only accepts values of type X
case x: X
// only accepts values matches by pattern
case x # pattern
// only accepts a value equal to the value X (upper case here makes a difference)
case X
// only accepts a value equal to the value of x
case `x`
// only accept a tuple of the same arity
case (x, y, ..., z)
// only accepts if extractor(value) returns true of Some(Seq()) (some empty sequence)
case extractor()
// only accepts if extractor(value) returns Some something
case extractor(x)
// only accepts if extractor(value) returns Some Seq or Tuple of the same arity
case extractor(x, y, ..., z)
// only accepts if extractor(value) returns Some Tuple2 or Some Seq with arity 2
case x extractor y
// accepts if any of the patterns is accepted (patterns may not contain assignable identifiers)
case x | y | ... | z
Now, extractors are the methods unapply or unapplySeq, the first returning Boolean or Option[T], and the second returning Option[Seq[T]], where None means no match is made, and Some(result) will try to match result as described above.
So there are all kinds of syntactic alternatives here, which just aren't possible without the use of one of the three constructions where pattern matches may happen. You may able to emulate some of the features, like value equality and extractors, but not all of them.
Patterns can also be used in for expressions. Your code sample
a match {
case b => // do stuff
case _ => // do nothing
}
can then be expressed as
for(b <- Some(a)) //do stuff
The trick is to wrap a to make it a valid enumerator. E.g. List(a) would also work, but I think Some(a) is closest to your intended meaning.
The best I can come up with is this:
def matches[A](a:A)(f:PartialFunction[A, Unit]) = f.isDefinedAt(a)
if (matches(a){case ... =>}) {
//do stuff
}
This won't win you any style points though.
Kim's answer can be “improved” to better match your requirement:
class AnyWrapper[A](wrapped: A) {
def matches(f: PartialFunction[A, Unit]) = f.isDefinedAt(wrapped)
}
implicit def any2wrapper[A](wrapped: A) = new AnyWrapper(wrapped)
then:
val a = "a" :: Nil
if (a matches { case "a" :: Nil => }) {
println("match")
}
I wouldn't do it, however. The => }) { sequence is really ugly here, and the whole code looks much less clear than a normal match. Plus, you get the compile-time overhead of looking up the implicit conversion, and the run-time overhead of wrapping the match in a PartialFunction (not counting the conflicts you could get with other, already defined matches methods, like the one in String).
To look a little bit better (and be less verbose), you could add this def to AnyWrapper:
def ifMatch(f: PartialFunction[A, Unit]): Unit = if (f.isDefinedAt(wrapped)) f(wrapped)
and use it like this:
a ifMatch { case "a" :: Nil => println("match") }
which saves you your case _ => line, but requires double braces if you want a block instead of a single statement... Not so nice.
Note that this construct is not really in the spirit of functional programming, as it can only be used to execute something that has side effects. We can't easily use it to return a value (therefore the Unit return value), as the function is partial — we'd need a default value, or we could return an Option instance. But here again, we would probably unwrap it with a match, so we'd gain nothing.
Frankly, you're better off getting used to seeing and using those match frequently, and moving away from this kind of imperative-style constructs (following Madoc's nice explanation).
Does scala have an equivalent to scheme's cond?
I guess you're looking for match (or just simply if/else if/else).
case class Paired(x: Int, y: Int)
def foo(x: Any) = x match {
case string : String => println("Got a string")
case num : Int if num < 100 => println("Number less than 100")
case Paired(x,y) => println("Got x and y: " + x + ", " + y)
case unknown => println("??: " + unknown)
}
The first two case statements show type based pattern matching. The third shows the use of an Extractor to break data down into constituent parts and to assign those parts to variables. The third shows a variable pattern match which will match anything. Not shown is the _ case:
case _ => println("what")
Which like the variable pattern match, matches anything, but does not bind the matched object to a variable.
The case class at the top is Scala shorthand for creating an extractor as well as the class itself.
Of course, neither match nor if does exactly the same thing as cond. One possibility is to do like this:
object Cond {
def apply(clauses: Iterable[(()=>Boolean, ()=>Any)]): Any = {
clauses find (_._1()) map (_._2()) getOrElse ()
}
}
This object accepts something Iterable where each item is a pair of a function returning Boolean and a function returning Any. It tries to find an item whose first function returns true, stops looking if it finds one, calls the second function on a found item and returns the result of that function (or () if none was found).
Examples:
val clauses = Seq(
({()=>false}, {()=>println("foo")}),
({()=>true}, {()=>println("bar")})
)
Cond(clauses)
def checkYear(year: Int) = {
Cond(Seq(
({()=>year % 400 == 0}, {()=>42}),
({()=>true}, {()=>{c:Char => (c.toString * 3)}})
))
}
ETA: Yes, I know it is ugly, but it works.
The most straightforward translation is to use pattern guards, although it requires some boilerplate. Pattern guards only work in a case pattern, and case only works in a match (unless we're writing a PartialFunction).
We can satisfy these conditions by matching a unit value against trivial cases:
;; Scheme
(cond
(foo bar)
(baz quux)
(t mydefault))
// Scala
() match {
case _ if foo => bar
case _ if baz => quux
case _ => mydefault
}
regarding the following scala code, functions m2a and m2b apparently differ only by the case of the parameter, ie abc vs Abc. This seems to make some difference in the result as per example below. When running it with a recent 2.8 compiler, it results in the following (I would have expected all true). Any insights would be appreciated.
m1=true
m2a=true
m2b=false
m3=true
code
package sample
import scala.xml._
object ParamTest extends Application {
def m1(n:Node, abc:String):Boolean = {
n == <id>{Text(abc)}</id>
}
def m2a(n:Node, Abc:String):Boolean = n match {
case <id>{Text(Abc)}</id> => true
case _ => false;
}
// why does this one not work?
def m2b(n:Node, abc:String):Boolean = n match {
case <id>{Text(abc)}</id> => true
case _ => false;
}
def m3(n:Node, abc:String):Boolean = n match {
case Elem(_,"id",_,_, c #_ *) => {
c contains Text(abc)
}
}
def runner(n:Node, f:(Node, String)=>Boolean):Boolean = {
f(n, "x") && !f(n, "y") && !f(n, "");
}
val x = <id>x</id>
println("m1="+runner(x, m1));
println("m2a="+runner(x, m2a));
println("m2b="+runner(x, m2b));
println("m3="+runner(x, m3));
}
The trick here is in how Scala handles variables in case expressions. Lowercase variables in case expressions are taken by the compiler to introduce new variables, which are then pattern matched against. Thus in method m2b, the method parameter "abc" is actually unused. The case expression variable "abc" will match any string, since it is not otherwise constrained. Thus "y" is successfully matched in the first case of m2b. Uppercase variables in case expressions do not introduce new variables, so in m2a the match behaves as you expected.
The easiest way to match against the value of a lowercase variable is to wrap it in backquotes. Thus
def m2b(n:Node, abc:String):Boolean = n match {
case <id>{Text(`abc`)}</id> => true
case _ => false;
}
will give you the results you expected.
In pattern matching, identifiers in the pattern that start with a lower-case letter are taken to be free pattern variables that can be bound to values in the target of the match. Those that start with upper-case letters are so-called stable identifiers and must already be bound in the match expression's context and the value of that binding must equal the subexpression of the match target at the point in that value corresponding to that stable identifier's placement within the pattern expression.
Additionally, and relevant in this particular example, pattern variables (the lower-case names) will shadow any existing binding of the same name that is in effect in the context of the match expression (including the expression supplying the match target value).