[Haskell-cafe] Transparent identity instances
wren ng thornton
wren at freegeek.org
Mon Nov 29 01:57:13 CET 2010
On 11/28/10 9:59 AM, Jafet wrote:
> But GHC does not accept type synonym instances unless they are fully applied.
That's precisely the problem, and why a newtype is used. More than GHC
implementation details, there's the deeper problem that allowing general
type-level functions causes decidability problems in type
checking/inference. Using a newtype with its explicit wrapping and
unwrapping solves the problem of inference by, essentially, adding type
annotations. Similar tricks are involved in making recursive types work.
More information about the Haskell-Cafe