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September 2012

45 participants
22 discussions

Constraint error related to type family and higher-rank type
by Tsuyoshi Ito 05 Sep '12

05 Sep '12

Hello, Can anyone please explain why the following code is rejected by GHC (7.4.1)? The same code is also available at https://gist.github.com/3606849. ----- {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE TypeFamilies #-} module Test1 where class C a b | b -> a data A = A data X = X data Y = Y type family TF b f :: (forall b. (C a b, TF b ~ Y) => b) -> X f _ = undefined u :: (C A b, TF b ~ Y) => b u = undefined v :: X v = f u -- This line causes an error (see below) ----- (1) GHC rejects this code with the following error message. Test1.hs:24:7: Could not deduce (C A b) arising from a use of `u' from the context (C a_c b, TF b ~ Y) bound by a type expected by the context: (C a_c b, TF b ~ Y) => b at Test1.hs:24:5-7 Possible fix: add (C A b) to the context of a type expected by the context: (C a_c b, TF b ~ Y) => b or add an instance declaration for (C A b) In the first argument of `f', namely `u' In the expression: f u In an equation for `v': v = f u (2) If I remove “TF b ~ Y” from the type of the argument of f and the type of u, then the code compiles. This suggests that the error message in (1) might not be the accurate description of the problem. (3) If I write “(f :: (forall b. (C A b, TF b ~ Y) => b) -> X)” instead of just “f” in the definition of v, then GHC reports a different error: Test1.hs:24:6: Cannot deal with a type function under a forall type: forall b. (C A b, TF b ~ Y) => b In the expression: f :: (forall b. (C A b, TF b ~ Y) => b) -> X In the expression: (f :: (forall b. (C A b, TF b ~ Y) => b) -> X) u In an equation for `v': v = (f :: (forall b. (C A b, TF b ~ Y) => b) -> X) u Item (3) might be related to a fixed Ticket #4310: http://hackage.haskell.org/trac/ghc/ticket/4310#comment:2 Best regards, Tsuyoshi

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RFC: Adding support for an API-"since"-version-attribute to Haddock?
by Herbert Valerio Riedel 05 Sep '12

05 Sep '12

Hello Haskellers, I've been wondering whether it might be useful to add a feature to Haddock similar to what can be found in other API documentation systems, specifically an optional parseable "since"-attribute, declaring the last package version when the associated definition/symbol was introduced or its semantics modified. Beyond giving a uniform/standard representation for this kind of information (e.g. if Haddock would render this information in a distinctive style suitable to give a visual hint for locating this information easily), this information could be used to generate an automated changelog entry (at least for the current package version), listing all definitions added or modified with the current version. Another application I could thinkg of would be to find the lowest version of a package supporting the currently used API from it (given the currently known-to-work package-version and a list of currently used symbols from that package). Would such an enhancement to Haddock be worthwhile or is it a bad idea? Has such a proposal come up in the past already? Are there alternative approaches to consider?

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