On Fri, Apr 27, 2012 at 08:39:23AM -0300, romildo@malaquias.DHCP-GERAL wrote:
On Thu, Apr 26, 2012 at 10:21:36AM +0200, José Pedro Magalhães wrote:
Hi Romildo,
If I understand correctly, you now want to add annotations to mutually-recursive datatypes. The annotations package supports that. Section 8 of our paper [1] gives an example of how to do that, and also Chapter 6 of Martijn's MSc thesis [2].
Let me know if these references do not answer your question.
I am reading Martijn's MSc thesis and trying some code he presents. In secton 5.1 he presents catamorphisms over fixed points.
The code I am trying is attached.
When evaluating the expression
cata exprEval (runExpr (1+2*3))
I am getting the following error:
No instance for (Functor ExprF) arising from a use of `cata' Possible fix: add an instance declaration for (Functor ExprF) In the expression: cata exprEval (runExpr (1 + 2 * 3)) In an equation for `it': it = cata exprEval (runExpr (1 + 2 * 3))
How should an instance of (Functor ExprF) be defined? It is not shown in the thesis. [...] type Id = String
data Op = Add | Sub | Mul | Div deriving (Show)
data ExprF r = Num Double | Var Id | Bin Op r r deriving (Show)
I could write the (Functor ExprF) instance: instance Functor ExprF where fmap f expr = case expr of Num n -> Num n Var v -> Var v Bin op x y -> Bin op (f x) (f y) Romildo