I'm going to use megaparsec for parsing a programming language for university project. However, I searched for finding a way to report multiple errors.
I know about withRecovery and I saw this issue but I didn't find about the case where errors happen on different positions.
for example in this java code :
class A
{
public get() // line 3 column 10
{
return x // line 5 column 22
}
}
There are error1 "expected type at line 3 column 10" and error2 "missing semicolon at line 5 column 22"
I know I can combine error messages with failure but how about multiple positions ? How do I do that ?
If you're sure about Alec's suggestion, but don't want to have Either (ParseError ...) (Either (ParseError ...) a), you can just use Control.Monad.join on that value to turn it into an Either (ParseError ...) a. Sorry if this wasn't too helpful
Related
This is a follow-up to my previous question.
I am finally able to reproduce the error here:
my #recentList = prompt("Get recentList: e.g. 1 2 3: ").words || (2,4,6);
say "the list is: ", #recentList;
for #recentList -> $x {
say "one element is: ", $x;
say "element type is: ", $x.WHAT;
say "test (1,2,3).tail(\"2\") : ", (1,2,3).tail("2");
say ( (10.rand.Int xx 10) xx 15 ).map: { #($_.tail($x)); };
}
And the results are ok as long as I use the default list by just hitting return at the prompt and not entering anything. But if I enter a number, it gives this error:
Get recentList: e.g. 1 2 3: 2
the list is: [2]
one element is: 2
element type is: (Str)
test (1,2,3).tail("2") : (2 3)
This type cannot unbox to a native integer: P6opaque, Str
in block at intType.p6 line 9
in block <unit> at intType.p6 line 5
If tail("2") works, why does tail($x) fail? Also, in my original code, tail($x.Int) wouldn't correct the problem, but it did here.
This is at best a nanswer. It is a thus-far failed attempt to figure out this problem. I may have just wandered off into the weeds. But I'll publish what I have. If nothing else, maybe it can serve as a reminder that the first three steps below are sensible ones; thereafter I'm gambling on my ability to work my way forward by spelunking source code when I would probably make much faster and more reliable progress by directly debugging the compiler as discussed in the third step.
OK, the first step was an MRE. What you've provided was an E that was fully R and sufficiently M. :)
Step #2 was increasing the M (golfing). I got it down to:
Any.tail('0'); # OK
Any.tail('1'); # BOOM
Note that it can be actual values:
1.tail('1'); # BOOM
(1..2).tail('1'); # BOOM
But some values work:
(1,2).tail('1'); # OK
Step #3 probably should be to follow the instructions in Playing with the code of Rakudo Perl 6 to track the compiler's execution, eg by sticking says in its source code and recompiling it.
You may also want to try out App::MoarVM::Debug. (I haven't.)
Using these approaches you'll have the power to track with absolute precision what the compiler does for any code you throw at it. I recommend you do this even though I didn't. Maybe you can figure out where I've gone wrong.
In the following I trace this problem by just directly spelunking the Rakudo compiler's source code.
A search for "method tail" in the Rakudo sources yielded 4 matches. For my golf the matching method is a match in core/AnyIterableMethods.pm6.
The tail parameter $n clearly isn't a Callable so the pertinent line that continues our spelunking is Rakudo::Iterator.LastNValues(self.iterator,$n,'tail').
A search for this leads to this method in core/Iterator.pm6.
This in turn calls this .new routine.
These three lines:
nqp::if(
n <= 0, # must be HLL comparison
Rakudo::Iterator.Empty, # negative is just nothing
explain why '0' works. The <= operator coerces its operands to numeric before doing the numeric comparison. So '0' coerces to 0, the condition is True, the result is Rakudo::Iterator.Empty, and the Any.tail('0') yields () and doesn't complain.
The code that immediately follows the above three lines is the else branch of the nqp::if. It closes with nqp::create(self)!SET-SELF(iterator,n,f).
That in turn calls the !SET-SELF routine, which has the line:
($!lastn := nqp::setelems(nqp::list, $!size = size)),
Which attempts to assign size, which in our BOOM case is '1', to $!size. But $!size is declared as:
has int $!size;
Bingo.
Or is it? I don't know if I really have correctly tracked the problem down. I'm only spelunking the code in the github repo, not actually running an instrumented version of the compiler and tracing its execution, as discussed as the sensible step #3 for trying to figure out the problem you've encountered.
Worse, when I'm running a compiler it's an old one whereas the code I'm spelunking is the master...
Why does this work?
(*,*).tail('1') # OK
The code path for this will presumably be this method. The parameter $n isn't a Callable so the code path will run thru the path that uses the $n in the lines:
nqp::unless(
nqp::istype($n,Whatever) || $n == Inf,
$iterator.skip-at-least(nqp::elems($!reified) - $n.Int)
The $n == Inf shouldn't be a problem. The == will coerce its operands to numerics and that should take care of $n being '1'.
The nqp::elems($!reified) - $n.Int shouldn't be a problem either.
The nqp ops doc shows that nqp::elems always returns an int. So this boils down to an int - Int which should work.
Hmm.
A blame of these lines shows that the .Int in the last line was only added 3 months ago.
So, clutching at straws, what happens if one tries:
(my int $foo = 1) - '1' # OK
Nope, that's not the problem.
It seems the trail has grown cold or rather I've wandered off the actual execution path.
I'll publish what I've got. Maybe someone else can pick it up from here or I'll have another go in a day or three...
I am the beginner of haskell. I want to delete some same functions in the same list and concatenate the two list get together.
For example:
db1 = ["David","worksfor.isa", "IBM" ]
db2 = ["David","isa'.worksfor'", "IBM"]
db3 = ["Tom","worksfor.isa", "IBM" ]
the program can be known that "isa'.worksfor' and "worksfor.isa" is the same String. And then use "Concat" to get the new db: db1 =["David","worksfor.isa", "IBM" ] and the others: db3 = ["Tom","worksfor.isa", "IBM" ]
(map (\(a,b,c) -> concat (map(\(a',b',c') -> if ( a b == b' a') then [] else [(a,b ++ "." ++ b',c')])))) ??????
I want to "split the string, if there are ' characters, reverse it, then remove ' characters and check for equivalence"
This should be a comment, but it is far too long:
I assume you find it hard to express yourself in English. I can relate to that; I find it hard myself. However, beyond English there are two other ways to communicate here:
Using precise technical terms.
Using several, diverse examples. A single example will not suffice, and several examples which are too similar give little information.
As for option 1, you are using the wrong terminology. It is not easy for me to see how can a list with 3 items can be considered a database (as hinted by the names db1, db2). Perhaps you wanted to use a list of triples?
[ ("David","isa'.worksfor'", "IBM") ]
You are not specific about what exactly do you want to concatenate, but the term concatenation always refers to an operation that must be "additive", i.e. length(x ++ y) == length(x) ++ length(y). This does not seem to be the case in your question.
Do you want a union of two databases (lists of triples) up to equivalence?
You want the program to understand that
"isa'.worksfor'" and "worksfor.isa" are the same string
But they are not. They might be equivalent strings. You can generally do that using a map operation, like you tried, but you should note that the character ' is not an operation over strings. So a b == b' a' does nothing close to what you want - it calls the function a on the variable b, and compares this with calling the function b' over the variable a'. I can only assume you want something like "split the string, if there are ' characters, reverse it, then remove ' characters and check for equivalence" but this is completely a guesswork.
To conclude:
Please explain in detail what is the general problem you are trying to solve. Try to find the precise terms; it is difficult, but this way you can learn.
Please add different examples of input and output
Please try to explain what have you tried and where are you stuck
As a last tip, maybe you want to solve this problem in a more forgiving language than Haskell (such as JavaScript, Python, Ruby, etc.)
Why is the function name repeated in
example:
lucky :: (Integral a) => a -> String
lucky 7 = "LUCKY NUMBER SEVEN!"
lucky x = "Sorry, you're out of luck, pal!"
when should I not be repeating function name? what is the meaning of it?
thanks
What you are seeing is pattern match in action.
I will show you another example:
test 1 = "one"
test 2 = "two"
test 3 = "three"
Demo in ghci:
ghci> test 1
"one"
ghci> test 2
"two"
ghci> test 3
"three"
ghci> test 4
"*** Exception: Non-exhaustive patterns in function test
So, when you call any function, the runtime system will try to match
the input with the defined function. So a call to test 3 will
initially check test 1 and since 1 is not equal to 3, it will
move on to the next definition. Again since 2 is not equal to 3,
it will move to the next defintion. In the next definiton since 3 is
equal to 3 it will return "three" String back. When you try to
pattern match something, which doesn't exist at all, the program
throws the exception.
This kind of pattern matching can be transformed to a case statement (and indeed, that's what compilers will normally do!):
lucky' n = case n of
7 -> "LUCKY NUMBER SEVEN!"
x -> "Sorry, you're out of luck, pal!"
Because the x isn't really used, you'd normally write _ -> "Sorry, ..." instead.
Note that this is not2 the same as
lucky'' n = if n==7 then ...
Equality comparison with (==) is in general more expensive1 than pattern matching, and also comes out uglier.
1 Why it's more expensive: suppose we have a big data structure. To determine that they are equal, the program will need to dig through both entire structures, make sure really all branches are equal. However, if you pattern match, you will just compare a small part you're interested in right now.
2 Actually, it is the same in the case, but just because the compiler has a particular trick for pattern matching on numbers: it rewrites it with (==). This is really special for Num types and not true for anything else. (Except if you use the OverloadedStrings extension.)
That definition of lucky uses "pattern matching", and equals (in this case)
lucky :: (Integral a) => a -> String
lucky a = if a == 7
then "LUCKY NUMBER SEVEN!"
else "Sorry, you're out of luck, pal!"
I assume you're looking at learn you a haskell. After that example, it says that
When you call lucky, the patterns will be checked from top to bottom and when it conforms to a pattern, the corresponding function body will be used.
So the first line indicates the type of the function, and later lines are patterns to check. Each line has the function name so the compiler knows you're still talking about the same function.
Think of it this way: When you write the expression lucky (a+b) or whatever, the compiler will attempt to replace lucky (a+b) with the first thing before the = in the function definition that "fits." So if a=3 and b=4, you get this series of replacements:
lucky (a+b) =
lucky (3+4) =
--pattern matching occurs...
lucky 7 =
"LUCKY NUMBER SEVEN!"
This is part of what makes Haskell so easy to reason about in practice; you get a system that works similarly to math.
after a lot of trial/error and the search function I am still somewhat clueless about an I-thought-simple-thing (as always, hrmpf):
I have a column in a data frame x$question and within that column, there is an expression 'A/V' every once in a while, and I simply want it to be changed to 'A / B'.
I tried a little here and there, and thought this should work:
x$question[agrep('A/V',x$question)]<-'A / B'
but I get the error:
In `[<-.factor`(`*tmp*`, agrep('A/V', :
invalid factor level, NAs generated
or I could do this
agrep('A/V','A / B', x$question).
But here I get the error:
Error in .amatch_bounds(max.distance) :
match distance components must be non-negative
Since I am quite out of ideas, I would be very thankful, if you had a suggestions, or maybe an even simpler way of replacing a string with another string.
Does this work?
gsub("A/V","A/B",x$question)
Example:
x<-c("A/V", "A/V", "A/V")
x<-gsub("A/V","A/B",x)
>x
[1] "A/B" "A/B" "A/B"
Note: You can use ifelse for that too.
> ifelse(x=="A/B","A/V",x)
[1] "A/V" "A/V" "A/V"
For example;
data TRAINING=AGAIN Int [TRAINING]
|RUN
|JUMP
|PUNCH Int
deriving (Eq,Show,Read)
is defined and I want that if the User enters something like:
"RUN, PUNCH 15, AGAIN 3 [JUMP, AGAIN 2 [PUNCH 20]]"
then the program should return
[RUN,PUNCH 15,AGAIN 3 [JUMP,AGAIN 2 [PUNCH 20]]]
So I wrote
fight :: String->[TRAINING]
fight xs=[read xs ::TRAINING]
but I am getting "no parse Exception". I am novice and I want to know what a "no parse Exception" is and how I can fix it ?
A no parse exception means that what you gave Haskell isn't the correct pattern for the instance of Read. In this case it's because list's are shown like this:
[<show element>,<show element>...]
And you're missing the outer brackets. Fixing it is as easy as seeing what the output should be:
Prelude> show [RUN,PUNCH 15,AGAIN 3 [JUMP,AGAIN 2 [PUNCH 20]]]
"[RUN,PUNCH 15,AGAIN 3 [JUMP,AGAIN 2 [PUNCH 20]]]"
So you need to surround the whole thing with []'s. Your function is right, you just have a slightly incorrect input string.
If you don't like this restriction, it may be time to just write a simple parser with Parsec or similar. Though this might be a bit challenging if you're totally new to Haskell.
In other words, following jozefg's answer:
fight xs = read xs ::[TRAINING]
and also:
"[RUN, PUNCH 15, AGAIN 3 [JUMP, AGAIN 2 [PUNCH 20]]]"