tagged-0.2.3.1: Provides newtype wrappers for phantom types to avoid unsafely passing dummy argumentsSource codeContentsIndex
Data.Tagged
Portabilitygeneralized newtype deriving
Stabilityexperimental
MaintainerEdward Kmett <ekmett@gmail.com>
Contents
Tagged values
Description
Synopsis
newtype Tagged s b = Tagged {
unTagged :: b
}
retag :: Tagged s b -> Tagged t b
untag :: Tagged s b -> b
tagSelf :: a -> Tagged a a
untagSelf :: Tagged a a -> a
asTaggedTypeOf :: s -> Tagged s b -> s
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"

Constructors
Tagged
unTagged :: b
show/hide Instances
retag :: Tagged s b -> Tagged t bSource

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 -> bSource
Alias for unTagged
tagSelf :: a -> Tagged a aSource
Tag a value with its own type.
untagSelf :: Tagged a a -> aSource
untagSelf is a type-restricted version of untag.
asTaggedTypeOf :: s -> Tagged s b -> sSource
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.
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