Identifiers in Nix are essentially strings. They can be constructed (and viewed) with the ident isomorphism. For the sake of convenience, Identifiers are an instance of the IsString class.

It is usually wise to only use identifiers of the form [a-zA-Z_][a-zA-Z0-9_'-]*, because these don't need quoting. Consequently, they can appear almost anywhere in a Nix expression (whereas quoted identifiers e.g. can't be used in function patterns). The methods of the Pretty class can be used to print an identifier with proper quoting:

>>> pPrint (ident # "test")
test
>>> pPrint (ident # "foo.bar")
"foo.bar"

The HasParser class allows parsing rendered identifiers even if they are quoted:

>>> parseM "Identifier" "hello" :: Maybe Identifier
Just (Identifier "hello")
>>> parseM "Identifier" "\"3rd party\"" :: Maybe Identifier
Just (Identifier "3rd party")

Warning: Identifiers may not contain '\0', but this is not checked during construction!

See also https://nix.dev/manual/nix/2.30/language/identifiers.html.

An isomorphism that allows conversion of Identifier from/to the standard String type via review.

>>> ident # "hello"
Identifier "hello"
>>> from ident # fromString "hello"
"hello"

Parsec parser for simple identifiers, i.e. those that don't need quoting. The parser is equivalent to the regular expression ^[a-zA-Z_][a-zA-Z0-9_'-]*$ which the Nix parser uses.

Note that this parser will accept keywords which would not be parsed as identifiers by Nix, see nixKeywords.

ReadP parser for quoted identifiers, i.e. those that do need quoting.

Checks whether a given string needs quoting when interpreted as an Identifier.

List of strings that are parseable as simple identifiers (see parseSimpleIdentifier) in isolation, but won't be accepted by Nix because keywords take precedence.

Helper function to quote a given identifier string if necessary. Usually, one should use the Pretty instance of Identifier instead.

>>> putStrLn (quote "abc")
abc
>>> putStrLn (quote "abc.def")
"abc.def"
>>> putStrLn (quote "$foo")
"$foo"
>>> putStrLn (quote "${foo}")
"\${foo}"