tagged-0.8.3: Haskell 98 phantom types to avoid unsafely passing dummy arguments

Copyright2009-2015 Edward Kmett
LicenseBSD3
MaintainerEdward Kmett <ekmett@gmail.com>
Stabilityexperimental
Portabilityportable
Safe HaskellSafe
LanguageHaskell98

Data.Tagged

Contents

Description

 

Synopsis

Tagged values

newtype Tagged s b Source

A Tagged s b value is a value b with an attached phantom type s. This can be used in place of the more traditional but less safe idiom of passing in an undefined value with the type, because unlike an (s -> b), a Tagged s b can't try to use the argument s as a real value.

Moreover, you don't have to rely on the compiler to inline away the extra argument, because the newtype is "free"

Tagged has kind k -> * -> * if the compiler supports PolyKinds, therefore there is an extra k showing in the instance haddocks that may cause confusion.

Constructors

Tagged 

Fields

unTagged :: b
 

Instances

Bifunctor (Tagged *) Source 
Monad (Tagged k s) Source 
Functor (Tagged k s) Source 
Applicative (Tagged k s) Source 
Foldable (Tagged k s) Source 
Traversable (Tagged k s) Source 
Generic1 (Tagged k s) Source 
Bounded b => Bounded (Tagged k s b) Source 
Enum a => Enum (Tagged k s a) Source 
Eq b => Eq (Tagged k s b) Source 
Floating a => Floating (Tagged k s a) Source 
Fractional a => Fractional (Tagged k s a) Source 
Integral a => Integral (Tagged k s a) Source 
(Data s, Data b) => Data (Tagged * s b) Source 
Num a => Num (Tagged k s a) Source 
Ord b => Ord (Tagged k s b) Source 
Read b => Read (Tagged k s b) Source 
Real a => Real (Tagged k s a) Source 
RealFloat a => RealFloat (Tagged k s a) Source 
RealFrac a => RealFrac (Tagged k s a) Source 
Show b => Show (Tagged k s b) Source 
Ix b => Ix (Tagged k s b) Source 
Generic (Tagged k s b) Source 
Monoid a => Monoid (Tagged k s a) Source 
NFData b => NFData (Tagged k s b) Source 
type Rep1 (Tagged k s) Source 
type Rep (Tagged k s b) Source 

retag :: Tagged s b -> Tagged t b Source

Some times you need to change the tag you have lying around. Idiomatic usage is to make a new combinator for the relationship between the tags that you want to enforce, and define that combinator using retag.

data Succ n
retagSucc :: Tagged n a -> Tagged (Succ n) a
retagSucc = retag

untag :: Tagged s b -> b Source

Alias for unTagged

tagSelf :: a -> Tagged a a Source

Tag a value with its own type.

untagSelf :: Tagged a a -> a Source

untagSelf is a type-restricted version of untag.

asTaggedTypeOf :: s -> tagged s b -> s Source

asTaggedTypeOf is a type-restricted version of const. It is usually used as an infix operator, and its typing forces its first argument (which is usually overloaded) to have the same type as the tag of the second.

witness :: Tagged a b -> a -> b Source

Conversion

proxy :: Tagged s a -> proxy s -> a Source

Convert from a Tagged representation to a representation based on a Proxy.

unproxy :: (Proxy s -> a) -> Tagged s a Source

Convert from a representation based on a Proxy to a Tagged representation.

tagWith :: proxy s -> a -> Tagged s a Source

Another way to convert a proxy to a tag.

Proxy methods GHC dropped

reproxy :: proxy a -> Proxy b Source

Some times you need to change the proxy you have lying around. Idiomatic usage is to make a new combinator for the relationship between the proxies that you want to enforce, and define that combinator using reproxy.

data Succ n
reproxySucc :: proxy n -> Proxy (Succ n)
reproxySucc = reproxy