trivial-constraint: Constraints that any type, resp. no type fulfills
Since GHC 7.4, constraints are first-class: we have the constraint
kind, and thus type-classes have a kind of form k -> Constraint
,
or k -> l -> m -> ... -> Constraint
for a multi-param type class.
Such type-level functions can be used as arguments to data types, or
as instances for other type classes.
For any given arity, the constraint-valued functions form a semigroup with respect to “constraint intersection”, which Haskell supports with tuple syntax, e.g.
type NewCstrt¹ a = (Cstrt¹₀ a, Cstrt¹₁ a)
means that NewCstrt¹ :: * -> Constraint
requires that for
any given parameter both Cstrt¹₀
and Cstrt¹₁
be fulfilled.
It is intuitive enough that this type-level semigroup can be extended
to a monoid, but out of the box this is only possible for arity 0,
i.e. for Cstrt⁰ :: Constraint
(Cstrt⁰, ()) ~ ((), Cstrt⁰) ~ Cstrt⁰
For higher arity, this would require type-level lambdas, like for
Cstrt² :: * -> * -> Constraint
(Cstrt², \a b -> ()) ~ (\a b -> (), Cstrt²) ~ Cstrt²
which is not valid Haskell syntax. It is easy enough to define the lambdas in an ad-hoc manner as
type Unconstrained² a b = ()
and then
(Cstrt², Unconstrained²) ~ (Unconstrained², Cstrt²) ~ Cstrt²
This library provides those trivial constraints in a single, documented place, and it uses classes instead of type-synonyms (which would be problematic when it comes to partial application). Arities 0-9 are provided.
They can be useful in any construction that is parameterised over a constrainer-class, when you do not wish to actually constrain the domain with it.
The other thing this library provides are the opposite classes,
i.e. \a b ... -> Impossible
, constraints which can never be
fulfilled. They are essentially dual to the Unconstrained
ones,
and can likewise be useful as parameters that should completely
“disable” a conditional feature.
Downloads
- trivial-constraint-0.7.0.0.tar.gz [browse] (Cabal source package)
- Package description (as included in the package)
Maintainer's Corner
For package maintainers and hackage trustees
Candidates
- No Candidates
Versions [RSS] | 0.1.0.0, 0.2.0.0, 0.3.0.0, 0.4.0.0, 0.5.0.0, 0.5.1.0, 0.6.0.0, 0.7.0.0 |
---|---|
Dependencies | base (>=4.5 && <5) [details] |
License | GPL-3.0-only |
Author | Justus Sagemüller |
Maintainer | jsag@hvl.no |
Category | Constraints |
Home page | https://github.com/leftaroundabout/trivial-constraint |
Source repo | head: git clone git://github.com/leftaroundabout/trivial-constraint.git |
Uploaded | by leftaroundabout at 2021-05-07T08:36:49Z |
Distributions | LTSHaskell:0.7.0.0, NixOS:0.7.0.0, Stackage:0.7.0.0 |
Reverse Dependencies | 5 direct, 7 indirect [details] |
Downloads | 7598 total (15 in the last 30 days) |
Rating | (no votes yet) [estimated by Bayesian average] |
Your Rating | |
Status | Docs available [build log] Last success reported on 2021-05-07 [all 1 reports] |