fcf-containers

Fcf-containers mimicks the containers package but for type-level computations. That is, we provide e.g. trees and maps. In addition to that, this package contains some other type-level computation utilities.

These methods are based on the idea given in the first-class-families -package, or Fcf shortly. Fcf is the main dependency of fcf-containers. As some of the methods fit badly under the name "fcf-containers", they might end up into the Fcf or some other package to be created. So stay tuned, be patient, check the TODO.md and send those PR's :)

Motivation for calculating things on type-level or on compile-time include

• increase the safety measures of runtime methods,
• pre-calculate complex things once on compile time and not every time the executable is run,
• provide users a way to choose between different algorithms for solving a problem based on problem instance properties (e.g. local vs network, or small vs large) known in advance.

Why fcf-like? The kind of signatures used for functions might be easier to read for some people and the ability to apply partially a function is nice tool to have. The techniques that allows this are defunctionalization, encoding the functions with empty data types and the use of open type family to Eval the constructed expressions.

If you have other motivations, please do let us know!

Note: some of the claims on the items in the above list are such that I believe/hope but really don't know at the moment nor do I know how check them. E.g. the matter of compile time vs run time. Yes, types are erased at compile time but do they still leave something into executables: simple check by comparing outputs of the orbit example and another program that has one method to print integer 42 and main, reveals that sizes are almost the same, but not exactly.

There are lot of open interesting questions. See TODO.md file. E.g. how combine these techniques with singletons-lib and related techniques.

Installation and building

First, get the repo with git clone and cd into the directory.

nix-shell 
cabal build 
cabal test 

The doc-tests both document and work as main testing mechanism for this lib.

If you don't use nix, cabal install fcf-containers should be enough. This package has almost as good number of dependencies as the first-class-families.

Example

See Orbits.hs. It shows how to solve a real problem, what PRAGMAs are probably needed etc.

cabal run orbits 

There is also another example that show how to use MapC, see Haiku.hs

cabal run haiku 

Random Notes

Partiality and anonymous functions

In the end, everything has to be total. We just post-pone the totality checking with defunctionalization in a way by trying to evaluate our functions as late as possible with the Eval function.

We don't have lambdas, but if you can write the helper function in point-free form, it might can be used directly without any global function definition. Remember, that (<=<) corresponds to term-level (.) and (=<<) to term-level function application ($). See also Maguire's book (Thinking with Types).

Conflicting family instance declarations

Transforming term-level Haskell code is relatively straigthforward. Often, local definitions in where and anonymous functions will be turned into separate helper functions.

Occasionally, the pattern matching is not quite enough. Please, consider

isPrefixOf              :: (Eq a) => [a] -> [a] -> Bool
isPrefixOf [] _         =  True
isPrefixOf _  []        =  False
isPrefixOf (x:xs) (y:ys)=  x == y && isPrefixOf xs ys

We could try to define it as

data IsPrefixOf :: [a] -> [a] -> Exp Bool
type instance Eval (IsPrefixOf '[] _) = 'True
type instance Eval (IsPrefixOf _ '[]) = 'False
type instance Eval (IsPrefixOf (x ': xs) (y ': ys)) =
         Eval ((Eval (TyEq x y)) && Eval (IsPrefixOf xs ys))

But ghc does not like this definition: the first two type instances are conflicting together. Instead, in these situations we can use a helper type family:

data IsPrefixOf :: [a] -> [a] -> Exp Bool
type instance Eval (IsPrefixOf xs ys) = IsPrefixOf_ xs ys

-- helper for IsPrefixOf
type family IsPrefixOf_ (xs :: [a]) (ys :: [a]) :: Bool where
    IsPrefixOf_ '[] _ = 'True
    IsPrefixOf_ _ '[] = 'False
    IsPrefixOf_ (x ': xs) (y ': ys) =
         Eval ((Eval (TyEq x y)) && IsPrefixOf_ xs ys)

Using If

If possible, try to avoid using Eval in the if-branches. For example, consider

    (If (Eval (s > 0) )
        ( 'Just '( a, s TL.- 1 ))
        'Nothing
    )

and

    (If (Eval (s > 0))
        (Eval (Pure ( 'Just '( a, s TL.- 1 ))))
        (Eval (Pure 'Nothing))
    )

Both compile and it is easy to end up in the latter form, especially if the branch is more complex than in this example.

The former, however, is much better as it doesn't have to evaluate both branches and is thus more efficient.

Other

The ghci and :kind! command are your friends!

Source also contains a lot of examples, see fcf-containers.

Happy :kinding!