If I try to compile it with FlexibleInstances I get:
<interactive>:5:10: error:
• The constraint ‘Foldable t’ is no smaller than the instance head
(Use UndecidableInstances to permit this)
• In the instance declaration for ‘(Traversable t)’
<interactive>:5:10: error:
• The constraint ‘Applicative t’
is no smaller than the instance head
(Use UndecidableInstances to permit this)
• In the instance declaration for ‘(Traversable t)’
<interactive>:5:10: error:
• Variable ‘a’ occurs more often
in the constraint ‘Monoid (t a)’ than in the instance head
(Use UndecidableInstances to permit this)
• In the instance declaration for ‘(Traversable t)’
Ok, so let's add UndecidableInstances, which results in:
<interactive>:7:10: error:
• Could not deduce (Monoid (t a0))
from the context: (Foldable t, Applicative t, Monoid (t a))
bound by an instance declaration:
forall (t :: * -> *) a.
(Foldable t, Applicative t, Monoid (t a)) =>
Traversable t
at <interactive>:7:10-69
The type variable ‘a0’ is ambiguous
• In the ambiguity check for an instance declaration
To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
In the instance declaration for ‘(Traversable t)’
Adding AllowAmbiguousTypes:
<interactive>:10:10: error:
• Could not deduce (Functor t)
arising from the superclasses of an instance declaration
from the context: (Foldable t, Applicative t, Monoid (t a))
bound by the instance declaration at <interactive>:10:10-69
Possible fix:
add (Functor t) to the context of the instance declaration
• In the instance declaration for ‘Traversable t’
<interactive>:10:10: error:
• Could not deduce (Monoid (t a0))
arising from a use of ‘Data.Traversable.$dmtraverse’
from the context: (Foldable t, Applicative t, Monoid (t a))
bound by the instance declaration at <interactive>:10:10-69
or from: Applicative f
bound by the type signature for:
traverse :: forall (f :: * -> *) a3 b.
Applicative f =>
(a3 -> f b) -> t a3 -> f (t b)
at <interactive>:10:10-69
The type variable ‘a0’ is ambiguous
Relevant bindings include
traverse :: (a3 -> f b) -> t a3 -> f (t b)
(bound at <interactive>:10:10)
• In the expression: Data.Traversable.$dmtraverse @t
In an equation for ‘traverse’:
traverse = Data.Traversable.$dmtraverse @t
In the instance declaration for ‘Traversable t’
<interactive>:10:10: error:
• Could not deduce (Monoid (t a1))
arising from a use of ‘Data.Traversable.$dmmapM’
from the context: (Foldable t, Applicative t, Monoid (t a))
bound by the instance declaration at <interactive>:10:10-69
or from: Monad m
bound by the type signature for:
mapM :: forall (m :: * -> *) a3 b.
Monad m =>
(a3 -> m b) -> t a3 -> m (t b)
at <interactive>:10:10-69
The type variable ‘a1’ is ambiguous
Relevant bindings include
mapM :: (a3 -> m b) -> t a3 -> m (t b)
(bound at <interactive>:10:10)
• In the expression: Data.Traversable.$dmmapM @t
In an equation for ‘mapM’: mapM = Data.Traversable.$dmmapM @t
In the instance declaration for ‘Traversable t’
<interactive>:10:10: error:
• Could not deduce (Monoid (t a2))
arising from a use of ‘Data.Traversable.$dmsequence’
from the context: (Foldable t, Applicative t, Monoid (t a))
bound by the instance declaration at <interactive>:10:10-69
or from: Monad m
bound by the type signature for:
sequence :: forall (m :: * -> *) a3.
Monad m =>
t (m a3) -> m (t a3)
at <interactive>:10:10-69
The type variable ‘a2’ is ambiguous
Relevant bindings include
sequence :: t (m a3) -> m (t a3) (bound at <interactive>:10:10)
• In the expression: Data.Traversable.$dmsequence @t
In an equation for ‘sequence’:
sequence = Data.Traversable.$dmsequence @t
In the instance declaration for ‘Traversable t’
<interactive>:10:113: error:
• Could not deduce (Monoid (t a3)) arising from a use of ‘mappend’
from the context: (Foldable t, Applicative t, Monoid (t a))
bound by the instance declaration at <interactive>:10:10-69
or from: Applicative f
bound by the type signature for:
sequenceA :: forall (f :: * -> *) a3.
Applicative f =>
t (f a3) -> f (t a3)
at <interactive>:10:77-85
• In the expression: mappend b (pure a)
In the second argument of ‘($)’, namely
‘\ b a -> mappend b (pure a)’
In the first argument of ‘foldl’, namely
‘(liftA2 $ \ b a -> mappend b (pure a))’