haskell - Compiling a type class, Queue, with an instance for lists, don't work due to incomplete inference in GHC 7.8 -
Trying to compile the following Haskell program:
{- # LANGUAGE multimartet class # -}
Remove C: - C -> (E, C) Blank :: C - & gt; Bull example queue [E] e where new = [] insert = () extract (x: x) = (x, xs) empty = ll list :: qi ci => C -> [E] toList queue | | Empty queue = [] list queue = first (before, rest) = remove from first queue: toList rest main = print (new ([[:: :: :: int])) < P> The following error: [1 to 1] Compilation Main (afff.hs, afff.o) afff.hs: 19: 16: arising from the use (QiCi1) Could not remove an empty ??? For reference (line CE) type bound by type sign: queue CE = & gt; C -> [E] at afff.hs: 18: 11-31 type variable â ???? E1â ???? It is unclear in the relevant binding line: C (bound at afff.hs: 19: 8) toList :: c -> [E] (binding on afff.hs: 19: 1) Note: A possible example is available: Example queue [e] defined on e-afff.hs: 12: 10 in Expression: An empty queue for an SMTM From a Pattern Protector list for the equation: ????: Empty Queue: List in an equation for a ????: toList queue | Empty queue = [] afff.hs: 22: 8: print an event (show A0) arising from the use of any one ??? Type variable 'a' ???? A0â ???? Note: There are several possible examples: Show example twice - defined in GHC. Floatio Example showing float - defined GACC. Example (see Integral A, A) = & gt; Show (GHC.RialArtioA) - Define GACC. In real ... plus 24 others in expression: print (new ([] :: [int])) for main in an equation ????: main = print (new ([] :: [int]) ) Afff.hs: 22: 15: (There is no example for qi ([int] -> a0) e0 is generated by the use of a new experiment. In the first impression "print", meaning "new" (new ([] :: [int]) in expression: print (new ([] :: [int])) for main in an equation ???? Main = print (new ([] :: [int]) returned shell 1 Why GHC is not able to remove "E1 = IT", if this is clearly the case is?
It is not obvious that (e1 ~ Int) is just So that (c ~ [int]) . To use MPTC, GHC should know every type of parameter for it. The type of empty refers to only one type of parameter, so it can never be used. On one side, I am very firmly of opinion that this is a bug, GHC does not immediately reject the definition of this class. It is clearly impossible to use, so why do you wait until you try to use it to reject it?
You think that just because the only one example matches the c type parameter you are okay, but you are not aware of GHC who is at the time of class resolution Examples are visible from It determines which category is needed, then it is checked for. In your case, it says, "I know what I want to be c , but .. I have no clue what I want e < / Code>. I can not tell which example I should use. "As I say on one side, this problem is obvious in class definition. The most direct way to resolve this is with a functional dependency, such as:
class Qi CE. C -> E where new :: insert c: e - & gt; C -> Remove C: - C -> (E, C) Blank :: C - & gt; Bool To use it, the Functionalandensis language extension is required. It specifies the compiler that the type is determined by type e specific type c . This means that it rejects a couple of instances where they have the same type as c but different types, e , and believe that if it Knows how to use c which is enough information to choose from.
Comments
Post a Comment