New to elm here, and at first it's driving me absolutely crazy not knowing the ins and outs of this picky language (even after reading a sh**load about it because it's just so different and finicky...I guess that's the nature of a functional lang) so when you try doing a simple thing it's like pulling hair at first.
I am getting the following error:
The right side of (==) is causing a type mismatch.
29| get 0 arrayOfValues == 'X'
^^^
(==) is expecting the right side to be a:
Maybe Char
But the right side is:
Char
Hint: With operators like (==) I always check the left side first. If it seems
fine, I assume it is correct and check the right side. So the problem may be in
how the left and right arguments interact.
Test:
it "blah blah blah" <|
let
someArray =
[ 'P', ' ' ]
in
expect (MyModule.doSomething someArray 'P') to equal 1
MyModule
doSomething : List Char -> Char -> Int
doSomething arrayOfValues symbol =
let
grid =
fromList arrayOfValues
found =
get 0 arrayOfValues == symbol
in
if found then
1
else
0
Now I'm assuming but not sure, that it's getting Nothing or something when trying to pull the first value out of my array but not sure. Maybe Char I assume is returning Nothing? donno, probably have other issues going on with it too.
I'd like to get the code above working, then refactor..I'm sure there's probably a more elegant way to code what I've coded above but first thing's first, fixing this error and understanding it better with the existing code. The error message while nice isn't that obvious to me as to how and what to handle. I have assumptions but not fully sure how to handle the behavior here of whatever is causing the issue.
Array.get returns a value wrapped in a Maybe because there might not be a value present at the specified index in the array. If you want to check whether the value at index 0 present and equal to 'X', you can compare to Just 'X':
get 0 arrayOfValues == Just 'X'
Like the error message says, the compiler found that the left side of == is a Maybe Char and the right is Char. You need to convert one to another to use ==. In this case, you probably want to change the right side as I suggested above.
Related
New to elm here, and at first it's driving me absolutely crazy not knowing the ins and outs of this picky language (even after reading a sh**load about it because it's just so different and finicky... I guess that's the nature of a functional lang) so when you try doing a simple thing it's like pulling hair at first.
I am getting the following error:
The right side of (==) is causing a type mismatch.
29| get 0 arrayOfValues == 'X'
^^^
(==) is expecting the right side to be a:
Maybe Char
But the right side is:
Char
Hint: With operators like (==) I always check the left side first. If it seems
fine, I assume it is correct and check the right side. So the problem may be in
how the left and right arguments interact.
Test:
it "blah blah blah" <|
let
someArray =
[ 'P', ' ' ]
in
expect (MyModule.doSomething someArray 'P') to equal 1
MyModule
doSomething : List Char -> Char -> Int
doSomething arrayOfValues symbol =
let
grid =
fromList arrayOfValues
found =
get 0 arrayOfValues == symbol
in
if found then
1
else
0
Now I'm assuming but not sure, that it's getting Nothing or something when trying to pull the first value out of my array but not sure. Maybe Char I assume is returning Nothing? donno, probably have other issues going on with it too.
I'd like to get the code above working, then refactor..I'm sure there's probably a more elegant way to code what I've coded above but first thing's first, fixing this error and understanding it better with the existing code. The error message while nice isn't that obvious to me as to how and what to handle. I have assumptions but not fully sure how to handle the behavior here of whatever is causing the issue.
Unique feature of the elm is certainty. Any variable (which is not of type maybe) will have a value of the defined type for sure.
But when it comes to array or list, it becomes uncertain if the array has an element on index "i". There may be an element and there may not be.
Hence elm has concept of Maybe,
so conceptually
Maybe String = [ Just "string_value" | Nothing ]
the alias for the Array.get is
get : Int -> Array a -> Maybe a
it takes
Int - index and
Array a - array of data type of array element
as parameters and returns
Maybe a - again a is the data type of array element
consider an example
array =
fromList ["one", "two"]
val1 =
get 0 array -- will return 'Just "one"'
val2 =
get 3 array -- will return 'Nothing', since the element does not exists
this way you will always have to handle both the situations, when you have a value and when you don't
case val1 of
Nothing ->
-- Raise some error message
Just val ->
-- `val` is the actual element/value found
and if you always need a default value, you can use
Maybe.withDefault "default_string" val1
this will always return a string value and will return "default_string" when the value is nothing otherwise the actual found value
Okay, what I really wanted to do is, I have an Array and I want to choose a random element from it. The obvious thing to do is get an integer from a random number generator between 0 and the length minus 1, which I have working already, and then applying Array.get, but that returns a Maybe a. (It appears there's also a package function that does the same thing.) Coming from Haskell, I get the type significance that it's protecting me from the case where my index was out of range, but I have control over the index and don't expect that to happen, so I'd just like to assume I got a Just something and somewhat forcibly convert to a. In Haskell this would be fromJust or, if I was feeling verbose, fromMaybe (error "some message"). How should I do this in Elm?
I found a discussion on the mailing list that seems to be discussing this, but it's been a while and I don't see the function I want in the standard library where the discussion suggests it would be.
Here are some pretty unsatisfying potential solutions I found so far:
Just use withDefault. I do have a default value of a available, but I don't like this as it gives the completely wrong meaning to my code and will probably make debugging harder down the road.
Do some fiddling with ports to interface with Javascript and get an exception thrown there if it's Nothing. I haven't carefully investigated how this works yet, but apparently it's possible. But this just seems to mix up too many dependencies for what would otherwise be simple pure Elm.
(answering my own question)
I found two more-satisfying solutions:
Roll my own partially defined function, which was referenced elsewhere in the linked discussion. But the code kind of feels incomplete this way (I'd hope the compiler would warn me about incomplete pattern matches some day) and the error message is still unclear.
Pattern-match and use Debug.crash if it's a Nothing. This appears similar to Haskell's error and is the solution I'm leaning towards right now.
import Debug
fromJust : Maybe a -> a
fromJust x = case x of
Just y -> y
Nothing -> Debug.crash "error: fromJust Nothing"
(Still, the module name and description also make me hesitate because it doesn't seem like the "right" method intended for my purposes; I want to indicate true programmer error instead of mere debugging.)
Solution
The existence or use of a fromJust or equivalent function is actually code smell and tells you that the API has not been designed correctly. The problem is that you're attempting to make a decision on what to do before you have the information to do it. You can think of this in two cases:
If you know what you're supposed to do with Nothing, then the solution is simple: use withDefault. This will become obvious when you're looking at the right point in your code.
If you don't know what you're supposed to do in the case where you have Nothing, but you still want to make a change, then you need a different way of doing so. Instead of pulling the value out of the Maybe use Maybe.map to change the value while keeping the Maybe. As an example, let's say you're doing the following:
foo : Maybe Int -> Int
foo maybeVal =
let
innerVal = fromJust maybeVal
in
innerVal + 2
Instead, you'll want this:
foo : Maybe Int -> Maybe Int
foo maybeVal =
Maybe.map (\innerVal -> innerVal + 2) maybeVal
Notice that the change you wanted is still done in this case, you've simply not handled the case where you have a Nothing. You can now pass this value up and down the call chain until you've hit a place where it's natural to use withDefault to get rid of the Maybe.
What's happened is that we've separated the concerns of "How do I change this value" and "What do I do when it doesn't exist?". We deal with the former using Maybe.map and the latter with Maybe.withDefault.
Caveat
There are a small number of cases where you simply know that you have a Just value and need to eliminate it using fromJust as you described, but those cases should be few and far between. There's quite a few that actually have a simpler alternative.
Example: Attempting to filter a list and get the value out.
Let's say you have a list of Maybes that you want the values of. A common strategy might be:
foo : List (Maybe a) -> List a
foo hasAnything =
let
onlyHasJustValues = List.filter Maybe.isJust hasAnything
onlyHasRealValues = List.map fromJust onlyHasJustValues
in
onlyHasRealValues
Turns out that even in this case, there are clean ways to avoid fromJust. Most languages with a collection that has a map and a filter have a method to filter using a Maybe built in. Haskell has Maybe.mapMaybe, Scala has flatMap, and Elm has List.filterMap. This transforms your code into:
foo : List (Maybe a) -> List a
foo hasAnything =
let
onlyHasRealValues = List.filterMap (\x -> x) hasAnything
in
onlyHasRealValues
I am reading someone else sql and his code was like this
There is view called user_v with column path as Array
select * from user_v where 'USER_TYPE'=path[2]
can't i use
path[2] = 'USER_TYPE'
This is a precaution taken by some programmers in languages where assignment and comparison can be easily confused, such as C or PHP, where the following statement looks innocent:
if ( $foo = 1 )
but it is actually assigning 1 to $foo, and the if will always evaluate to true (in PHP, at least, where 1 is true). What was meant was if ( $foo == 1 ).
If you reverse the arguments, the error becomes obvious sooner:
if ( 1 = $foo ) # SYNTAX ERROR
if ( 1 == $foo ) # Desired behaviour
This is sometimes known as "yoda coding", and is in the coding standards of Wordpress, for example.
See also: Why put the constant before the variable in a comparison?
In SQL, there is less chance of such a muddle, since although = can mean either assignment or comparison, there are rarely situations where a typo would select the wrong meaning.
However, if the coding standards for every other language used by a project mandate it, it would make sense to reinforce the habit by also using it in SQL, since I can't think of a specific reason not to write it that way.
There is no difference at all.
It's psychology.
You would want to read someone else's code out laud and say:
Where my column equals 2.
When you read:
Where 2 equals my column
you have to stop for a while, return, explain it to yourself.
We maintain all of these rules that seem rubish at first glance just to make other people lives easier.
i am having a little trouble with printf specifiers...so before asking you guys i read almost everything onC++Reference page, but couldnt fix the problem, and since i am new at c i cant even understand the problem, its most likely a syntax error but i can't find it...
for(i = 1; i <= 10; i++) {
printf("\n%d.%s%n",i,names[i-1],offset);
printf("%*s%.2f TL",10-offset," ",prices[i-1]);
}
so basically i have this code to print a list, and i want the prices to start from the same column.
For e.g:
water 1.00
oj 1.00
and the logic behind my code (incase it's not obvious, i can't tell if it is) is:
print id number and name, count how many chars we've written so far and assign it to offset.
print (starting column of price list)-offset spaces before price
once i couldn't get the result i want, i checked and found out that offset is 3 for all names which is not the case(and no value is assigned to offset before this procedure).
Thanks for any kind of help !
PS: This is a practice code just to get better at using specifiers efficiently.
edit:
so i did this :
for(i=1;i<=10;i++)
{
printf("%d.%s%n",i,names[i-1],&offset);
printf("%*s%.2f TL\n",10-offset," ",prices[i-1]);
}
but what i get as a result is huge empty black command screen.
The %n format specifier requires a pointer. Your code is missing the & operator for offset:
printf("\n%d.%s%n",i,names[i-1],&offset);
The good ol' C interface doesn't know what types you supply to printf so it doesn't complain and happily reads the 4 byte integer value of offset on the stack as a memory location -> core dump.
Actually, g++ with -Wall does warn. So
hd1 has a point here because C++ output is type safe (even though it's a pain);
Heed thy warnings.
When you use %n in a printf format, the corresponding parameter must be a pointer. printf will store the information in the int you point it to.
Assuming you've declared int offset somewhere, you should use &offset as the last argument in your printf call.
While we're here, allow me to comment on this excerpt:
printf("\n
ARGH NO! Newline is a terminator. It goes at the end of a line, not the beginning.
so i did this :
for(i=1;i<=10;i++)
{
printf("%d.%s%n",i,names[i-1],&offset);
printf("%*s%.2f TL\n",10-offset," ",prices[i-1]);
}
but what i get as a result is huge empty black command screen.
edit: Can you guys try this and tell me if you get normal results? I can't understand the mistake occuring, so i can't get past it...Maybe some other examples will lead me to where is the mistake.
In VB.NET, there's no == operator for comparison, so the = operator serves that purpose as well as assignment. I have a function, and I want it to return the boolean result of a comparison, without storing that result in a variable:
Private Function foo() As Boolean
Dim bar As Integer = 1
Return bar = 2
End Function
Returns: False
OK, but what's the value of bar?
Private Function foo() As KeyValuePair(Of Boolean, Integer)
Dim bar As Integer = 1
Return New KeyValuePair(Of Boolean, Integer)(bar = 2, bar)
End Function
Returns: False, 1
It looks like = will perform a comparison when the statement context demands it, but is this guaranteed? That is, can I be sure that bar will never be set to 2 in this situation?
Also, I know that VB.NET doesn't allow chained inline assignments, which may be for the best. Does this odd = behavior cause any other quirks I should be aware of?
You cannot do in-line assignments in VB, Assignment is an explicit statement:
[Let] <<target-reference>> = <<value-expression>>
The Let is optional and implicit, and hardly ever used anymore. The general rule that you can use to distinguish the [Let] command from equality testing is that for Let, no other keyword may come before the target-reference in the statement. AFAIK, in all cases of = as equality testing, there is one or more other keywords that precede it in the statement.
In your first example, the keyword Return precedes your =, so it's an equality test, and not an assignment.
In your first example you can do either:
Return 2
or
bar = 2
Return bar
As for your question "OK, but what's the value of bar?", bar still equals one.
= in VB cause no quirks. It works exactly as documented, and it always has (including its predecessor, BASIC back to 1968).
If you are starting to code in VB (coming from a language like C#), you should start getting used to the peculiar VB way of doing things; which is based on the idea: as simple and intuitive for the programmer as possible. "If assignation and comparison happen always in different contexts, why not using the same operator and let the context define its exact meaning?" -> VB-way of seeing things. "No, different realities have to be accounted for by different operators. End of the discussion" -> C#-way. :)
Is this reliable? Can you blindly trust on these not-always-clear-for-a-programmer bits? Sure, VB.NET peculiarities are highly-reliable and trustworthy. You can always use = (or Is on some contexts, but VS would tell you) and be completely sure that the code will do what is expected. But the question is: are you sure that you write exactly what you want?
This last question is what, perhaps, is more criticable of VB and what might give some problems to programmers from other languages: the higher the flexibility, the more likely is that you make an error; mainly if you are used to a different format.
Regarding the chained inline assignments, I honestly don't see its true utility (and never use them in C#). Regarding other differences with respect to C#, there are plenty of them; in some cases, I think that the C# approach is better; other times, the VB.NET one. On readability/length of code, I can refer to the With Statement I have always found somehow useful which is not present in C#.
One way to have 100% sure that the expression will be evaluated as an boolean expression is to use ()
e.g
Dim a = 2
Return (a = 1)
Since you cannot set a value to a variable wihtin the parenthesis.
What i want to say is: on an return statament for example you cant assing a value to a variable so, even if you use
a = 1
The compilator knows that this expression only can be an boolean expression.
The same to the if statament and so on..
Heh back in QB45 days we used to exploit the fact that "True" was the numeric value -1. So you would see code like x = 1 - x * (x < 6) (translation: increment x, but reset to 1 when it gets to 6)