Why read cannot resolve a type with one argument?
matrix cannot be resolved to a type
greeting cannot be resolved to a type
jxdatepicker cannot be resolved to a type
annotationconfigcontextloader cannot be resolved to a type
query cannot be resolved to a type
item cannot be resolved to a type
node cannot be resolved to a type
The Haskell tutorial states that:
by looking at the type signature of
read :: Read a => String -> a
it follows that GHCI has no way of knowing which type we want in return when running
ghci> read "4"
Why is it necessary to provide a second value from which GHCI can extract a type to compare with?
Wouldn't it be feasible to check a single value against all possible types of the
I think you have a (rather common among beginners - I had it myself) misunderstanding of what type classes are. The way Haskell works is logically incompatible with "check[ing] a single value against all possible types of the Read typeclass". Instance selection is based on types. Only types.
You should not think of
read as a magical function that can return many types. It's actually a huge family of functions, and the type is used to select which member of the family to use. It's that direction of dependence that matters. Classes create a case where values (usually functions, but not always) - the things that exist at run time - are chosen based on types - the things that exist at compile time.
You're asking "Why not the other direction? Why can't the type depend on the value?", and the answer to that is that Haskell just doesn't work that way. It wasn't designed to, and the theory it was based on doesn't allow it. There is a theory for that (dependent types), and there are extensions being added to GHC that support an increasing set of feature that do some aspect of dependent typing, but it's not there yet.
And even if it was, this example would still not work the way you want. Dependent types still need to know what type something is. You couldn't write a magical "returns anything" version of
read. Instead, the type for
read would have to involve some function that calculates the type from the value, and inherently only works for the closed set of types that function can return.
Those last two paragraphs are kind of an aside, though. The important part is that classes are ways to go from types to values, with handy compiler support to automatically figure it out for you most of the time. That's all they were designed to do, and it's all that they can do. There are advantages to this design, in terms of ease of compilation, predictability of behavior (open world assumption), and ability to optimize at compile time.
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Wouldn't it be feasible to check a single value against all possible types of the Read typeclass?
Doing that would yield the same result;
read "4" can potentially be anything that can be read from a
String, and that's what
Prelude> :t read "4" read "4" :: Read a => a
Until you actually do the parsing, the
Read a => a represents a potential parsing result. Remember that typeclasses being open means that this could potentially be any type, depending on the presence of the instances.
It's also entirely possible that multiple types could share the same
Read textual representation, which brings me to my next point...
If you wanted to check what type the string can be parsed as, that would at the very least require resolving the ambiguity between multiple types that could accept the given input; which means you'd need to know those types beforehand, which
Read can't do. And even if you did that, how do you propose such value be then used? You'd need to pack it into something, which implies that you need a closed set again.
All in all,
read signature is as precise it can be, given the circumstances.
Java class "cannot be resolved to a type", The problem is that TeamLeader appears to be an inner class (hard to tell, your indentation is bad), and since it's not static, you can't instantiate it by itself. The problem lies in the fact that "T" in the context of the compiler could be a type like any other. Using letters T U V are conventions that we use for generics, but you could just as easily write List<SomeClass> instead.
Not meant as an answer, but this wouldn't fit into a comment cleanly.
In ghci, if you simply do a
read "5", then ghci is going to need some help figuring out what you want it to be. However, if that result is being used somewhere, ghci (and Haskell in general) can figure out the type. For (a silly) example:
add1 :: Int -> Int add1 i = i + 1 five = read "5" six = add1 five
In that case, there's no need to annotate the read with a type signature, because ghc can infer it from the fact that
five is being used in a function that only takes an Int. If you added another function with a different signature that also tried to use
five, you'd end up with a compile error:
-- Adding this to our code above -- Fails to compile add1Integer :: Integer -> Integer add1Integer i = i + 1 sixAsInteger = add1Integer five
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- What concrete type do you propose
read "5"should have? It's not so simple to say
- It's worse than that. Usually you don't use
read "5", because you'd just write
5. What concrete type should
- Error reading / parsing is the same result as success. For example, in cases where I expect a number, and the user has transmitted something else, then I would like to receive an parsing error, and not something else.
- The type signatures are not really similar at all.
showis polymorphic in its argument type - which is something quite normal. But
readis polymorphic in its result type, which means without either being told explicitly by a type signature, or inference from the other functions you use the result with, the compiler can't figure out what you want.
- You say, "Why is it necessary to provide a second value?". This question confuses me. It is not necessary -- and indeed, in most settings, not even allowed -- to provide a second value. For example,
read "4" 3will give you a missing instance error (in the absence of certain very unorthodox additional code). Can you give an example of what you mean, since you don't seem to be using standard terminology here?
- Well, maybe it is very naive but when the value of the
Stringis "4", all the potential types that
arepresents in the
readsignature are constrained. What i would expect is a list returning all possible types that could match:
- @Legaph What about
- As the OP is beginner, let's also say that
Integerare two diffferent types.
intwe know in c/c++/java/c#, which will overflow if we get hight that
Integeris a boundless/limitless integer: it is less efficient, but cannot overflow, since it has no maximum number limit.