fcf-containers alternatives and similar packages
Based on the "Other" category
Do you think we are missing an alternative of fcf-containers or a related project?
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.
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
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
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.
(<=<) corresponds to term-level
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)
If possible, try to avoid using
Eval in the if-branches.
For example, consider
(If (Eval (s > 0) ) ( 'Just '( a, s TL.- 1 )) 'Nothing )
(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.
:kind! command are your friends!
Source also contains a lot of examples, see fcf-containers.