T3406.hs:11:6:
The type variables ‘a, b’
should be bound by the pattern signature ‘ItemColID a b’
but are actually discarded by a type synonym
To fix this, expand the type synonym
[Note: I hope to lift this restriction in due course]
In the pattern: x :: ItemColID a b
In an equation for ‘get’:
get (x :: ItemColID a b) = x :: ItemColID a b