Hi Adam,
thank you for your quick and detailed answer! I think I understand how to
construct evidence for typeclass constraints now. But trying to apply this,
I still have some problems.
I have something along the following lines:
class Polymonad m n p where
-- Functions
instance Polymonad Identity Identity Identity where
-- Implementation
-- Further instances and some small chunk of code involving them:
The code implies the following constraint:
Polymonad Identity n_abpq Identity
As the ambiguity error I get says, when trying to compile this: There is
only one matching instance (the one above, lets call
it $fPolymonadIdentityIdentityIdentity).
So my plugin tries to tell GHC to use that instance. As far as I understand
it, since the parameters of $fPolymonadIdentityIdentityIdentity are no type
variables and there is no superclass it should be as easy as saying:
EvDFunApp $fPolymonadIdentityIdentityIdentity [] []
But when I run this with -dcore-lint I get the following error message:
*** Core Lint errors : in result of Desugar (after optimization) ***
<no location info>: Warning:
In the expression: >>
@ Identity
@ Any
@ Identity
$fPolymonadIdentityIdentityIdentity
@ ()
@ ()
(idOp @ Bool True)
(>>=
@ Identity
@ Identity
@ Any
$fPolymonadIdentityIdentityIdentity
@ Char
@ ()
(return
@ Char @ Identity
$fPolymonadIdentityIdentityIdentity (C# 'a'))
(\ _ [Occ=Dead] ->
return @ () @ Identity
$fPolymonadIdentityIdentityIdentity ()))
Argument value doesn't match argument type:
Fun type:
Polymonad Identity Any Identity =>
forall a_abdV[sk] b_abdW[sk].
Identity a_abdV[sk] -> Any b_abdW[sk] -> Identity b_abdW[sk]
Arg type: Polymonad Identity Identity Identity
Arg: $fPolymonadIdentityIdentityIdentity
What am I missing? Why doesn't the argument type "Polymonad Identity
Identity Identity" match the first argument of the function type "Polymonad
Identity Any Identity => forall a_abdV[sk] b_abdW[sk]. Identity a_abdV[sk]
-> Any b_abdW[sk] -> Identity b_abdW[sk]". Why is the type variable
translated to "Any"?
Best,
Jan
2015-02-25 13:35 GMT+00:00 Adam Gundry
Hi Jan,
Yes, unfortunately the meaning of EvTerm is a weak point of the current typechecker plugins story; it rather requires one to understand how GHC's constraint solver produces evidence. There are lots of papers on evidence for equality constraints in System FC, but typeclass constraints are generally ignored as they are just datatypes at the FC level.
Let me try to give you some idea of what EvDFunApp means, then hopefully those with more knowledge of the GHC internals can correct me...
If you write a class instance, e.g.
instance (Show a, Show b) => Show (T a b) where ...
then GHC generates a dfun (short for "dictionary function", I guess)
$fShowT :: forall a b . (Show a, Show b) => Show (T a b)
where Show is treated as a record data type containing a dictionary of methods for the class. At the core level, this is a normal term-level function (albeit with a strange name).
Now when the typechecker has a constraint to solve, say
Show (T Int Bool),
it produces evidence for this by applying $fShowT to the appropriate types and to evidence for the superclass constraints, in this case something like
$fShowT @Int @Bool $fShowInt $fShowBool
where I'm using @ for type application. This is represented in EvTerm as
EvDFunApp $fShowT [Int, Bool] [ EvDFunApp $fShowInt [] [] , EvDFunApp $fShowBool [] [] ]
Thus the [Type] is the list of kinds/types at which to instantiate the dfun, and the [EvTerm] is the list of evidence terms to which it must be applied. Obviously this application should be well-typed, and -dcore-lint will complain if it is not.
For typechecker plugins, it would be nice if we could write arbitrary core expressions as evidence, but this hasn't yet been implemented (partially because most of the examples so far solve equality constraints, rather than typeclass constraints).
Hope this helps,
Adam
On 25/02/15 10:55, Jan Bracker wrote:
Hello,
I am trying to use the new type checker plugins [1] that are implemented in head.
When successful a plugin has to return a [(EvTerm, Ct)] for the solved constraints. The documentation on EvTerms is scarce [2,3,4] and I could not find papers that explain them (many talk about 'evidence', but they never get concrete).
So far I have figured out that "EvDFunApp DFunId [Type] [EvTerm]" selects a certain instance to be used for a constraint, though I don't know what the list of EvTerms in the end is for. I am also a bit unclear on how the "[Type]" is used.If I turn on '-dcore-lint' I get errors. So I still seem to be using it wrong.
I have also asked in IRC, but did not get a response to my question.
I am sorry if this is the wrong mailing list to ask. If there is a more apropriate place just point it out.
Best, Jan
[1]: https://ghc.haskell.org/trac/ghc/wiki/Plugins/TypeChecker [2]: http://haskell.inf.elte.hu/docs/7.11.20150225.noWin32/html/libraries/ghc-7.1... [3]: http://haskell.inf.elte.hu/docs/7.11.20150225.noWin32/html/libraries/ghc-7.1... [4]: http://haskell.inf.elte.hu/docs/7.11.20150225.noWin32/html/users_guide/compi...
-- Adam Gundry, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/