GHC/Coercible
This page contains additional information about Coercible
and augments the documentation and the ICFP 2014 paper. This is a feature that first appeared in GHC 7.8.1 and will likely evolve futher.
The problem
Given a newtype
newtype HTML = MkHTML String
we can convert between HTML
and String
with
toHTML :: String -> HTML
toHTML s = MkHTML s
fromHTML :: HTML -> String
fromHTML (MkHTML s) = s
and these conversions are free, i.e. they have no run-time cost.
But how do we get from [String]
to [HTML]
? We can write
toHTMLs :: [String] -> [HTML]
toHTMLs = map MkHTML
but the execution of map
incurs a cost at run-time.
Using Coercible
The solution available since GHC-7.8.1 is to use coerce
from the module Data.Coerce
:
import Data.Coerce
toHTMLs :: [String] -> [HTML]
toHTMLs = coerce
It works like unsafeCoerce
, i.e. has no run-time cost, but the type checker ensures that it really is safe to use it. If you use it illegally like in
wrong :: [Int -> [Bool]
wrong = coerce
you will get an error message:
Could not coerce from ‘Int’ to ‘Bool’ because ‘Int’ and ‘Bool’ are different types. arising from a use of ‘coerce’ In the expression: coerce In an equation for ‘wrong’: wrong = coerce
The type of coerce
is Coercible a b => a -> b
, and the instances of the “type class” Coercible
(which behaves almost like a regular type class) ensure that Coercible s t
is only solvable if s
and t
have the same run-time representation.
Interesting things to note
Using newtypes internally and externally differently
You can unwrap a newtype using coerce
only if the corresponding constructor is in scope. This allows you to do free conversions in your own library (where the constructor is in scope), while controlling what others can do using role annotation:
This newtype has a phantom parameter, but the role annotation allows users of the library to coerce NT s
to NT t
only if Coercible s t
holds:
newtype NT a = MkNT ()
type role NT representational
Nevertheless, as long as the constructor MkNT
is in scope, we can do coerce :: NT Bool -> NT Int
, if we wish to do so.
(This does not yet work in GHC-7.8, as a bug in GHC was fixed only later.)