| Copyright | (C) 2021 Edward Kmett |
|---|---|
| License | BSD-style (see the file LICENSE) |
| Maintainer | Edward Kmett <ekmett@gmail.com> |
| Stability | provisional |
| Portability | portable |
| Safe Haskell | Safe |
| Language | Haskell2010 |
Data.Functor.Contravariant.Conclude
Description
Documentation
class Decide f => Conclude f where Source #
The contravariant analogue of
Plus
. Adds on to
Decide
the ability
to express a combinator that rejects all input, to act as the dead-end.
Essentially
Decidable
without a superclass constraint on
Divisible
.
If one thinks of
f a
as a consumer of
a
s, then
conclude
defines
a consumer that cannot ever receive
any
input.
Conclude acts as an identity with
decide
, because any decision that
involves
conclude
must necessarily
always
pick the other option.
That is, for, say,
decidef xconcluded
f
is the deciding function that picks which of the inputs of
decide
to direct input to; in the situation above,
f
must
always
direct all
input to
x
, and never
concluded
.
Mathematically, a functor being an instance of
Decide
means that it is
"monoidal" with respect to the contravariant "either-based" Day
convolution described in the documentation of
Decide
. On top of
Decide
, it adds a way to construct an "identity"
conclude
where
decide f x (conclude q) == x
, and
decide g (conclude r) y == y
.
Since: 5.3.6