[Haskell-cafe] Re: HList error with hFoldr

[oleg at okmij.org]

Denis Bueno wrote:

> For the moment my HLists only contain Ints.
...
> -- Why should the following generate an error?
> testApplyDistSum = hFoldr ApplyDistSum 0 ((4,4) .*. hNil)

Here is the problem: in Haskell, 4 is not an Int. (4::Int) is an Int,
but just 4 is a _polymorphic_ constant of the type Num a => a. Because
it is polymorphic and so does not have any defined type, the type 
checker cannot chose an appropriate instance for ApplyDistSum. The
error message says this directly:

>    No instance for (Apply ApplyDistSum ((t, t1), r1) r)

Note the appearance of lower-case identifiers t, t1, r1, and r in the
error message. They stand for the types that the type checker cannot
figure out. When you use hFoldr in a larger expression, the types
often become constrained by other components of the expression and so
type variables become instantiated. In the simple cases as above,
there is no information available to instantiate the type variables,
hence the error. You have to give the type-checker hints in the form
of type annotations:

 testApplyDistSum = hFoldr ApplyDistSum (0::Int) ((4::Int,4::Int) .*. hNil)

In general, it is possible to avoid the type annotation on the
accumulator (in this case, 0). The type checker could figure out the
type of the result. But that requires specifying functional
dependencies. The type annotations might be easier to start
with. Incidentally, it is generally a good idea in Haskell to write
Int constants as (1::Int), explicitly specifying their type. Here's a
bit elaborated example:

{-# OPTIONS -fglasgow-exts #-}

module B where

import HList

class (Num i) => MetricSpace e i where
    dist :: e -> e -> i

instance Num i => MetricSpace Int i where
    x `dist` y = fromIntegral $ abs (y - x)

instance Floating i => MetricSpace Float i where
    x `dist` y = realToFrac $ abs (y - x)

data ApplyDistSum = ApplyDistSum
instance (MetricSpace e r) => Apply ApplyDistSum ((e, e), r) r where
    apply _ (p, v) = v + uncurry dist p

testApplyDistSum = hFoldr ApplyDistSum (0::Double) ((4::Int,5::Int) .*. hNil)
testApplyDistSum1 = hFoldr ApplyDistSum (0::Double) (
   (4::Float,5.1::Float) .*. (4::Int,5::Int) .*. hNil)