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.
First, get the repo with git clone and cd into the directory.
nix-shell
cabal build
cabal test
If you are flakes user, nix develop -c zsh (or without that -c zsh) works as well.
The doc-tests both document and work partly as testing mechanism for this lib. Please
do note that we are moving away from the doc-tests and building the testing modules
under test directory.
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.
The
test directory
contains a lot of useful examples. There you will find out, how to apply most
of the methods: examples that only work on "compile time", and examples on how
to get the results from type level calculations to value level (see the Reflect
test module and the fromType method applications, also in some other testing
modules).
These are a bit larger examples, but not yet too large: somewhat convenient small module size. Idea was to take an algorithm, and convert it to a type-level computation, or in some other way give an example of methods that this library provides.
See Orbits.hs. It shows how to solve a real problem, what PRAGMAs are probably needed etc.
cabal run orbits
There is also another Advent of Code problem, see the Carbcombat file.
To see, how to use MapC, take a look at Haiku.hs
cabal run haiku
Please, do take a look of the notes made for the Helsinki Haskell meetup on 11th January 2023 notes and the associated code examples.
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).
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)
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.
The ghci and :kind! command are your friends!
Source also contains a lot of examples, see fcf-containers. The examples will be left near the code, even thou the doctest runs will be removed and replaced with real tests at the test-directory.
Happy :kinding!