I've been trying next purescript ver 0.12.0-rc1.
I have a question how to use new feature 'instance chain'.
in my understanding the instance chain provides a feature to be able to specify instance resolving order explicitly. this solves avoid instance definition overlapping.
so I suppose it may work:
class A a
class B b
class C c where
c :: c -> String
instance ca :: A a => C a where
c = const "ca"
else
instance cb :: B b => C b where
c = const "cb"
data X = X
instance bx :: B X
main :: forall eff. Eff (console :: CONSOLE | eff) Unit
main = logShow $ c X
but cannot compiled.
what is not correctly?
or what is the instance chain usage?
result:
Error found:
in module Main
at src/Main.purs line 23, column 8 - line 23, column 20
No type class instance was found for
Main.A X
while applying a function c
of type C t0 => t0 -> String
to argument X
while inferring the type of c X
in value declaration main
where t0 is an unknown type
Even with instance chains matching is still done on the head of an instance. There is no "backtracking" when any of constraint fails for the chosen instance.
Your instances are completely overlapping on the head, so your first instance always matches before second one and it fails because there is no A instance for X.
Instance chains allows you to define explicit ordering of instance resolution without relying on for example alphabetical ordering of names etc. (as it was done till 0.12.0 version - please check the third paragraph here). For example you can define this overlapping scenario:
class IsRecord a where
isRecord :: a -> Boolean
instance a_isRecordRecord :: IsRecord (Record a) where
isRecord _ = true
instance b_isRecordOther :: IsRecord a where
isRecord _ = false
as
instance isRecordRecord :: IsRecord (Record a) where
isRecord _ = true
else instance isRecordOther :: IsRecord a where
isRecord _ = false
I hope it compiles - I don't have purs-0.12.0-rc yet ;-)
Related
Suppose we have a record type that is homogeneous.
type RecI = { a :: Int, b :: Int, c :: Int, d :: Int, e :: Int }
We want to get from it type with the same keys but different value type:
type RecS = { a :: String, b :: String, c :: String, d :: String, e :: String }
Is it possible to get RecS type without explicitly defining all the keys from RecI?
And the second part of the question, what is the best way to implement mapping function from one type to another:
mapItoS :: (Int -> String) -> RecI -> RecS
?
To get a free-ish conversion from Int to String at type level, just give your record a parameter, then instantiate it with Int to get RecI and with String to get RecS:
type Rec a = { a :: a, b :: a, c :: a, d :: a, e :: a }
type RecI = Rec Int
type RecS = Rec String
To implement mapItoS, you can first convert to a Foreign.Object using fromHomogeneous, then map the function over it, then convert back to the record.
Unfortunately there is no toHomogeneous function, because in general you can't be sure that the Foreign.Object actually contains all required keys. But no matter: in this particular case you can be sure that it does, so you can get away with unsafeCoerce:
mapItoS :: forall a b. (a -> b) -> Rec a -> Rec b
mapItoS f = fromHomogeneous >>> map f >>> unsafeCoerce
A small self plug which is strictly relevant to the question :-P I've just published a library which provides many instances which allow a PureScripter to work with homogeneous Record and Variant:
https://pursuit.purescript.org/packages/purescript-homogeneous
I think it should have much better inference than solutions like heterogeneous. Please check it out and let me know what do you think.
Well, just simplified as possible:
There is a function that takes functor and does whatever
sToInt :: ∀ a s. Functor s => s a -> Int
sToInt val = unsafeCoerce val
Usage of this function with functor S which param (v) is functor too.
-- declare date type S that is functor
data S (v :: Type -> Type) a = S (v a)
instance functorS :: Functor v => Functor (S v) where
map f (S a) = S (map f a)
sV :: ∀ v a. S v a
sV = unsafeCoerce 1
sss :: Int
sss = sToInt sV -- get the error here
No type class instance was found for
Data.Functor.Functor t2
The instance head contains unknown type variables. Consider adding a type annotation.
while applying a function sToInt
of type Functor t0 => t0 t1 -> Int
to argument sV
while checking that expression sToInt sV
has type Int
in value declaration sss
where t0 is an unknown type
t1 is an unknown type
t2 is an unknown type
So it doesn't like S Functor instance has v param Functor constraint, I wonder why getting this error and how to fix it for this case.
This doesn't have to do with v or with the specific shape of S. Try this instead:
sV :: forall f a. Functor f => f a
sV = unsafeCoerce 1
sss :: Int
sss = sToInt sV
You get a similar error.
Or here's an even more simplified version:
sV :: forall a. a
sV = unsafeCoerce 1
sss :: Int
sss = sToInt sV
Again, same error.
The problem is that sToInt must get a Functor instance as a parameter (that's what the Functor s => bit in its type signature says), and in order to pick which Functor instance to pass, the compiler needs to know the type of the value. Like, if it's Maybe a, it will pass the Functor Maybe instance, and if it's Array a, it will pass the Functor Array instance, and so on.
Usually the type can be inferred from the context. For example when you say map show [1,2,3], the compiler knows that map should come from Functor Array, because [1,2,3] :: Array Int.
But in your case there is nowhere to get that information: sV can return S v for any v, and sToInt can also take any functor type. There is nothing to tell the compiler what the type should be.
And the way to fix this is obvious: if there is no context information for the compiler to get the type from, you have to tell it what the type is yourself:
sss :: Int
sss = sToInt (sV :: S Maybe _)
This will be enough for the compiler to know that v ~ Maybe, and it will be able to construct a Functor (S Maybe) instance and pass it to sToInt.
Alternatively, if you want the consumer of sss to decide what v is, you can add an extra dummy parameter to capture the type, and require that the consumer pass in a Functor v instance:
sss :: forall v. Functor v => FProxy v -> Int
sss _ = sToInt (sV :: S v _)
ddd :: Int
ddd = sss (FProxy :: FProxy Maybe)
In Haskell you can do this with visible type applications instead of FProxy, but PureScript, sadly, doesn't support that yet.
Even more alternatively, if sToInt doesn't actually care for a Functor instance, you can remove that constraint from it, and everything will work as-is:
sToInt :: forall s a. s a -> Int
sToInt a = unsafeCoerce a
sV :: forall v a. S v a
sV = unsafeCoerce 1
sss :: Int
sss = sToInt sV
This works because PureScript allows for ambiguous (aka "unknown") types to exist as long as they're not used for selecting instances.
Is there any way to do something like
first = {x:0}
second = {x:1,y:1}
both = [first, second]
such that both is inferred as {x::Int | r} or something like that?
I've tried a few things:
[{x:3}] :: Array(forall r. {x::Int|r}) -- nope
test = Nil :: List(forall r. {x::Int|r})
{x:1} : test -- nope
type X r = {x::Int | r}
test = Nil :: List(X) -- nope
test = Nil :: List(X())
{x:1} : test
{x:1, y:1} : test -- nope
Everything I can think of seems to tell me that combining records like this into a collection is not supported. Kind of like, a function can be polymorphic but a list cannot. Is that the correct interpretation? It reminds me a bit of the F# "value restriction" problem, though I thought that was just because of CLR restrictions whereas JS should not have that issue. But maybe it's unrelated.
Is there any way to declare the list/array to support this?
What you're looking for is "existential types", and PureScript just doesn't support those at the syntax level the way Haskell does. But you can roll your own :-)
One way to go is "data abstraction" - i.e. encode the data in terms of operations you'll want to perform on it. For example, let's say you'll want to get the value of x out of them at some point. In that case, make an array of these:
type RecordRep = Unit -> Int
toRecordRep :: forall r. { x :: Int | r } -> RecordRep
toRecordRep {x} _ = x
-- Construct the array using `toRecordRep`
test :: Array RecordRep
test = [ toRecordRep {x:1}, toRecordRep {x:1, y:1} ]
-- Later use the operation
allTheXs :: Array Int
allTheXs = test <#> \r -> r unit
If you have multiple such operations, you can always make a record of them:
type RecordRep =
{ getX :: Unit -> Int
, show :: Unit -> String
, toJavaScript :: Unit -> Foreign.Object
}
toRecordRep r =
{ getX: const r.x
, show: const $ show r.x
, toJavaScript: const $ unsafeCoerce r
}
(note the Unit arguments in every function - they're there for the laziness, assuming each operation could be expensive)
But if you really need the type machinery, you can do what I call "poor man's existential type". If you look closely, existential types are nothing more than "deferred" type checks - deferred to the point where you'll need to see the type. And what's a mechanism to defer something in an ML language? That's right - a function! :-)
newtype RecordRep = RecordRep (forall a. (forall r. {x::Int|r} -> a) -> a)
toRecordRep :: forall r. {x::Int|r} -> RecordRep
toRecordRep r = RecordRep \f -> f r
test :: Array RecordRep
test = [toRecordRep {x:1}, toRecordRep {x:1, y:1}]
allTheXs = test <#> \(RecordRep r) -> r _.x
The way this works is that RecordRep wraps a function, which takes another function, which is polymorphic in r - that is, if you're looking at a RecordRep, you must be prepared to give it a function that can work with any r. toRecordRep wraps the record in such a way that its precise type is not visible on the outside, but it will be used to instantiate the generic function, which you will eventually provide. In my example such function is _.x.
Note, however, that herein lies the problem: the row r is literally not known when you get to work with an element of the array, so you can't do anything with it. Like, at all. All you can do is get the x field, because its existence is hardcoded in the signatures, but besides the x - you just don't know. And that's by design: if you want to put anything into the array, you must be prepared to get anything out of it.
Now, if you do want to do something with the values after all, you'll have to explain that by constraining r, for example:
newtype RecordRep = RecordRep (forall a. (forall r. Show {x::Int|r} => {x::Int|r} -> a) -> a)
toRecordRep :: forall r. Show {x::Int|r} => {x::Int|r} -> RecordRep
toRecordRep r = RecordRep \f -> f r
test :: Array RecordRep
test = [toRecordRep {x:1}, toRecordRep {x:1, y:1}]
showAll = test <#> \(RecordRep r) -> r show
Passing the show function like this works, because we have constrained the row r in such a way that Show {x::Int|r} must exist, and therefore, applying show to {x::Int|r} must work. Repeat for your own type classes as needed.
And here's the interesting part: since type classes are implemented as dictionaries of functions, the two options described above are actually equivalent - in both cases you end up passing around a dictionary of functions, only in the first case it's explicit, but in the second case the compiler does it for you.
Incidentally, this is how Haskell language support for this works as well.
Folloing #FyodorSoikin answer based on "existential types" and what we can find in purescript-exists we can provide yet another solution.
Finally we will be able to build an Array of records which will be "isomorphic" to:
exists tail. Array { x :: Int | tail }
Let's start with type constructor which can be used to existentially quantify over a row type (type of kind #Type). We are not able to use Exists from purescript-exists here because PureScript has no kind polymorphism and original Exists is parameterized over Type.
newtype Exists f = Exists (forall a. f (a :: #Type))
We can follow and reimplement (<Ctrl-c><Ctrl-v> ;-)) definitions from Data.Exists and build a set of tools to work with such Exists values:
module Main where
import Prelude
import Unsafe.Coerce (unsafeCoerce)
import Data.Newtype (class Newtype, unwrap)
newtype Exists f = Exists (forall a. f (a :: #Type))
mkExists :: forall f a. f a -> Exists f
mkExists r = Exists (unsafeCoerce r :: forall a. f a)
runExists :: forall b f. (forall a. f a -> b) -> Exists f -> b
runExists g (Exists f) = g f
Using them we get the ability to build an Array of Records with "any" tail but we have to wrap any such a record type in a newtype before:
newtype R t = R { x :: Int | t }
derive instance newtypeRec :: Newtype (R t) _
Now we can build an Array using mkExists:
arr :: Array (Exists R)
arr = [ mkExists (R { x: 8, y : "test"}), mkExists (R { x: 9, z: 10}) ]
and process values using runExists:
x :: Array [ Int ]
x = map (runExists (unwrap >>> _.x)) arr
Suppose you are using the FlexibleInstances extension and have the class
class C a where
f :: a b -> Maybe b
how would you implement it for a list of lists of a datatype. In particular, how would the type be written. The only thing I could find is how to do it for a single list, but not a list of lists or lists of any other datatypes.
This works:
instance C [] where
...
But this doesn't
data D = ...
instance C [[D]] where
...
How can I express something like this?
You need a newtype
class C a where
f :: a b -> b -- the class before the OP edited
newtype LL a = LL [[a]]
instance C LL where
f (LL xss) = ...
However, it is impossible to write a completely meaningful instance, since if the lists-of-lists is empty, it is impossible to extract an element. the best we could do is
instance C LL where
f (LL xss) = case concat xss of
(x:_) -> x
_ -> error "f: no elements"
I'm not sure if that is a good idea.
As an alternative, you could use type families or functional dependencies. Here's a solution with type families.
{-# LANGUAGE TypeFamilies, FlexibleInstances #-}
class C a where
type T a
f :: a -> Maybe (T a)
instance C [[b]] where
type T [[b]] = b
f xss = case concat xss of
[] -> Nothing
(x:_) -> Just x
I'm having trouble with classes in haskell.
Basically, I have an algorithm (a weird sort of graph-traversal algorithm) that takes as input, among other things, a container to store the already-seen nodes (I'm keen on avoiding monads, so let's move on. :)). The thing is, the function takes the container as a parameter, and calls just one function: "set_contains", which asks if the container... contains node v. (If you're curious, another function passed in as a parameter does the actual node-adding).
Basically, I want to try a variety of data structures as parameters. Yet, as there is no overloading, I cannot have more than one data structure work with the all-important contains function!
So, I wanted to make a "Set" class (I shouldn't roll my own, I know). I already have a pretty nifty Red-Black tree set up, thanks to Chris Okasaki's book, and now all that's left is simply making the Set class and declaring RBT, among others, as instances of it.
Here is the following code:
(Note: code heavily updated -- e.g., contains now does not call a helper function, but is the class function itself!)
data Color = Red | Black
data (Ord a) => RBT a = Leaf | Tree Color (RBT a) a (RBT a)
instance Show Color where
show Red = "r"
show Black = "b"
class Set t where
contains :: (Ord a) => t-> a-> Bool
-- I know this is nonesense, just showing it can compile.
instance (Ord a) => Eq (RBT a) where
Leaf == Leaf = True
(Tree _ _ x _) == (Tree _ _ y _) = x == y
instance (Ord a) => Set (RBT a) where
contains Leaf b = False
contains t#(Tree c l x r) b
| b == x = True
| b < x = contains l b
| otherwise = contains r b
Note how I have a pretty stupidly-defined Eq instance of RBT. That is intentional --- I copied it (but cut corners) from the gentle tutorial.
Basically, my question boils down to this: If I comment out the instantiation statement for Set (RBT a), everything compiles. If I add it back in, I get the following error:
RBTree.hs:21:15:
Couldn't match expected type `a' against inferred type `a1'
`a' is a rigid type variable bound by
the type signature for `contains' at RBTree.hs:11:21
`a1' is a rigid type variable bound by
the instance declaration at RBTree.hs:18:14
In the second argument of `(==)', namely `x'
In a pattern guard for
the definition of `contains':
b == x
In the definition of `contains':
contains (t#(Tree c l x r)) b
| b == x = True
| b < x = contains l b
| otherwise = contains r b
And I simply cannot, for the life of me, figure out why that isn't working. (As a side note, the "contains" function is defined elsewhere, and basically has the actual set_contains logic for the RBT data type.)
Thanks! - Agor
Third edit: removed the previous edits, consolidated above.
You could also use higher-kinded polyphormism. The way your class is defined it sort of expects a type t which has kind *. What you probably want is that your Set class takes a container type, like your RBT which has kind * -> *.
You can easily modify your class to give your type t a kind * -> * by applying t to a type variable, like this:
class Set t where
contains :: (Ord a) => t a -> a -> Bool
and then modify your instance declaration to remove the type variable a:
instance Set RBT where
contains Leaf b = False
contains t#(Tree c l x r) b
| b == x = True
| b < x = contains l b
| otherwise = contains r b
So, here is the full modified code with a small example at the end:
data Color = Red | Black
data (Ord a) => RBT a = Leaf | Tree Color (RBT a) a (RBT a)
instance Show Color where
show Red = "r"
show Black = "b"
class Set t where
contains :: (Ord a) => t a -> a -> Bool
-- I know this is nonesense, just showing it can compile.
instance (Ord a) => Eq (RBT a) where
Leaf == Leaf = True
(Tree _ _ x _) == (Tree _ _ y _) = x == y
instance Set RBT where
contains Leaf b = False
contains t#(Tree c l x r) b
| b == x = True
| b < x = contains l b
| otherwise = contains r b
tree = Tree Black (Tree Red Leaf 3 Leaf) 5 (Tree Red Leaf 8 (Tree Black Leaf 12 Leaf))
main =
putStrLn ("tree contains 3: " ++ test1) >>
putStrLn ("tree contains 12: " ++ test2) >>
putStrLn ("tree contains 7: " ++ test3)
where test1 = f 3
test2 = f 12
test3 = f 7
f = show . contains tree
If you compile this, the output is
tree contains 3: True
tree contains 12: True
tree contains 7: False
You need a multi-parameter type class. Your current definition of Set t doesn't mention the contained type in the class definition, so the member contains has to work for any a. Try this:
class Set t a | t -> a where
contains :: (Ord a) => t-> a-> Bool
instance (Ord a) => Set (RBT a) a where
contains Leaf b = False
contains t#(Tree c l x r) b
| b == x = True
| b < x = contains l b
| otherwise = contains r b
The | t -> a bit of the definition is a functional dependency, saying that for any given t there is only one possible a. It's useful to have (when it makes sense) since it helps the compiler figure out types and reduces the number of ambiguous type problems you often otherwise get with multi-parameter type classes.
You'll also need to enable the language extensions MultiParamTypeClasses and FunctionalDependencies at the top of your source file:
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}
The error means that the types don't match. What is the type of contains? (If its type is not something like t -> a -> Bool as set_contains is, something is wrong.)
Why do you think you shouldn't roll your own classes?
When you write the instance for Set (RBT a), you define contains for the specific type a only. I.e. RBT Int is a set of Ints, RBT Bool is a set of Bools, etc.
But your definition of Set t requires that t be a set of all ordered a's at the same time!
That is, this should typecheck, given the type of contains:
tree :: RBT Bool
tree = ...
foo = contains tree 1
and it obviously won't.
There are three solutions:
Make t a type constructor variable:
class Set t where
contains :: (Ord a) => t a -> a-> Bool
instance Set RBT where
...
This will work for RBT, but not for many other cases (for example, you may want to use a bitset as a set of Ints.
Functional dependency:
class (Ord a) => Set t a | t -> a where
contains :: t -> a -> Bool
instance (Ord a) => Set (RBT a) a where
...
See GHC User's Guide for details.
Associated types:
class Set t where
type Element t :: *
contains :: t -> Element t -> Bool
instance (Ord a) => Set (RBT a) where
type Element (RBT a) = a
...
See GHC User's Guide for details.
To expand on Ganesh's answer, you can use Type Families instead of Functional Dependencies. Imho they are nicer. And they also change your code less.
{-# LANGUAGE FlexibleContexts, TypeFamilies #-}
class Set t where
type Elem t
contains :: (Ord (Elem t)) => t -> Elem t -> Bool
instance (Ord a) => Set (RBT a) where
type Elem (RBT a) = a
contains Leaf b = False
contains (Tree c l x r) b
| b == x = True
| b < x = contains l b
| otherwise = contains r b