Hello, everyone.
Considering, I have a class:
class Flt a where
allows :: FltOpts -> [a]
denies :: FltOpts -> [a]
crit :: a -> [a] -> Bool
flt :: FltOpts -> a -> Bool
flt opts a = allowed && not denied
where allowed = if null $ allows opts then True else a `crit` (allows opts)
denied = if null $ denies opts then False else a `crit` (denies opts)
I get error here:
• Could not deduce (Flt a1) arising from a use of ‘allows’
from the context: Flt a
bound by the class declaration for ‘Flt’
at .../.stack-work/intero/intero5319V42.hs:(31,1)-(38,97)
The type variable ‘a1’ is ambiguous
These potential instance exist:
instance Flt MyType
-- Defined at ...
• In the second argument of ‘($)’, namely ‘allows opts’
....................................................
As I understand, GHC can not deduce type if it's a return's value
(contraposition?). OK, but it knows its type: it is `[a]`! What is the
problem to keep `flt` method as a generic, i.e. without concreate type,
but only `[a]` ?
Second, I implemented instance:
instance Flt MyType where
allows = ...
denies = ...
flt opts a = allowed && not denied
where allowed = if null $ (allows opts::[MyType]) then True else a `crit` (allows opts)
denied = if null $ (denies opts::[MyType]) then False else a `crit` (denies opts)
and without this explicite type annotation of `allows opts` I get again
ambigous error. But why? GHC knows that `allows` returns `[a]` and `a`
is `MyType`, so `[a]` is `[MyType]`. Why I need to write it explicitly?
May be I need some extension here?
===
Best regards, Paul