Clear and concise answer.
Thank you.
On Thu, Apr 15, 2010 at 21:24, Daniel Fischer
Am Donnerstag 15 April 2010 19:19:15 schrieb Romulus:
Hello everyone,
I'm stuck with the definition of the helper for LAnd'. I expect :
land' :: ((LAnd' p) :<: (f p)) => Mu (f p) -> Mu (f p) -> Mu (f p) land' = \x y -> inject (LAnd' x y)
... but ghci 6.10.4 does not really like this definition...
Could not deduce (LAnd' p :<: f p1) from the context (LAnd' p1 :<: f p1) arising from a use of `inject' at PropSample.hs:128:16-33 Possible fix: add (LAnd' p :<: f p1) to the context of the type signature for `land'' or add an instance declaration for (LAnd' p :<: f p1) In the expression: inject (LAnd' x y) In the expression: \ x y -> inject (LAnd' x y) In the definition of `land'': land' = \ x y -> inject (LAnd' x y) Failed, modules loaded: none.
Does anybody have a clue for this problem ? I don't really understand where is the trouble actually.
The expression (Land' x y) can have the type (Land' q (Mu (f p))) for all q. But there's only an instance for the p used in Mu (f p) provided, so
Could not deduce (LAnd' p :<: f p1) from the context (LAnd' p1 :<: f p1)
The solution is to tell the compiler that this expression should have the type LAnd' p (Mu (f p)) for the f and p from the type signature, add
{-# LANGUAGE ScopedTypeVariables #-}
and modify land' to
land' :: forall f p. ((LAnd' p) :<: (f p)) => Mu (f p) -> Mu (f p) -> Mu (f p) land' = \x y -> inject (LAnd' x y :: LAnd' p (Mu (f p)))
to be greeted by
[1 of 1] Compiling Prop ( PropSample.hs, interpreted )
PropSample.hs:31:16: Warning: Declaration of `In' uses deprecated syntax Instead, use the form In :: {out :: f (Mu f)} -> Mu f Ok, modules loaded: Prop. *Prop>
by 6.12.1 and
$ ghci-6.10.3 PropSample GHCi, version 6.10.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Prop ( PropSample.hs, interpreted ) Ok, modules loaded: Prop. *Prop>
from the older GHC.
Cheers,
PS: haskellers rulez ;)
+1
[1] Swierstra, W. Data types à la carte J. Funct. Program. Cambridge University Press, 2008, 18, 423-436
[2] Knowles, K. First-Order Logic à la Carte The Monad.Reader, 2008, issue 11
-- Romuald THION Docteur level 1.0 - Great master access control, +3 against half-dead "la vie, c'est comme un jeu mal foutu dont on ne connait pas les règles et où il n'y a pas de sauvegardes"