It's a parsing infelicity. (Inside square brackets the parser knows not to expect a forall, whereas inside round parens it might.) Perhaps it should be more accepting in square brackets, and reject later. Which the current HEAD does -- actually [forall a. a->a] is ok in the HEAD, see our ICFP06 paper. Simon | -----Original Message----- | From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Neil | Mitchell | Sent: 03 July 2006 19:44 | To: Haskell Cafe | Subject: [Haskell-cafe] forall and a parse error | | Hi, | | I was experimenting with forall and higher rank types briefly, in particular: | | x :: [forall a . a] | | This is illegal because of: | http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions. html#universal-quantification | | Which is fine, however its surprising to compare the error messages: | | [forall a . a] | parse error on input `forall' | | [(forall a . a)] | Illegal polymorphic or qualified type: forall a. a | In the type signature: lst :: [(forall a. a)] | | In normal Haskell, I tend to view [x] as equivalent to [(x)] (provided | that x is not a tuple) but in this case it has a different meaning - | albeit both are erronous meanings. | | When running the example with Hugs, they both come out as syntax | errors - the first on the forall, the second on the closing square | bracket. | | Thanks | | Neil | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe