Question

In GHCI, if you run:

import Data.Binary
encode [1]

Everything works smoothly. But, if you then run:

import Data.Vector.Binary
encode [1]

you get an error message:

Overlapping instances for Binary [t0]
arising from a use of `encode’

Matching instances:

 instance (Data.Vector.Generic.Base.Vector v a, Binary a) =>
          Binary (v a)
   -- Defined in `Data.Vector.Binary'

 instance Binary a => Binary [a] -- Defined in `Data.Binary'

In the expression: encode [1]

In an equation for `it’: it = encode [1]

It seems like GHC thinks my list is a vector type instead of a list. Why is that?

Answer 1

Type class matching does not backtrack if the head matches. The instances head

 Binary (v a)

says “for any type v: * -> * and type a:* match Binary (v a).” Thus [] a matches. The instance in Data.Vector is probably more general than it should be. The OverlappingInstances extension allows you to resolve cases like this by selecting the “most specific instance” which in this case is clearly [a].

IMO, Haskell should provide a mechanism for controlling the scope of instances to help prevent problems like this, but it does not.