NimbleCSV v0.6.0 NimbleCSV behaviour View Source

NimbleCSV is a small and fast parsing and dumping library.

It works by building highly-inlined CSV parsers, designed to work with strings, enumerables and streams. At the top of your file (and not inside a function), you can define your own parser module:

NimbleCSV.define(MyParser, separator: "\t", escape: "\"")

Once defined, we can parse data accordingly:

iex> MyParser.parse_string "name\tage\njohn\t27"
[["john","27"]]

See the define/2 function for the list of functions that would be defined in MyParser.

Parsing

NimbleCSV is by definition restricted in scope to do only parsing (and dumping). For example, the example above discarded the headers when parsing the string, as NimbleCSV expects developers to handle those explicitly later. For example:

"name\tage\njohn\t27"
|> MyParser.parse_string
|> Enum.map(fn [name, age] ->
  %{name: name, age: String.to_integer(age)}
end)

This is particularly useful with the parse_stream functionality that receives and returns a stream. For example, we can use it to parse files line by line lazily:

"path/to/csv/file"
|> File.stream!(read_ahead: 100_000)
|> MyParser.parse_stream
|> Stream.map(fn [name, age] ->
  %{name: name, age: String.to_integer(age)}
end)

By default this library ships with NimbleCSV.RFC4180, which is the most common implementation of CSV parsing/dumping available using comma as separators and double-quote as escape. If you want to use it in your codebase, simply alias it to CSV and enjoy:

iex> alias NimbleCSV.RFC4180, as: CSV
iex> CSV.parse_string "name,age\njohn,27"
[["john","27"]]

Binary references

One of the reasons behind NimbleCSV performance is that it performs parsing by matching on binaries and extracting those fields as binary references. Therefore if you have a row such as:

one,two,three,four,five

NimbleCSV will return a list of ["one", "two", "three", "four", "five"] where each element references the original row. For this reason, if you plan to keep the parsed data around in the parsing process or even send it to another process, you may want to copy the data before doing the transfer.

For example, in the parse_stream example in the previous section, we could rewrite the Stream.map/2 operation to explicitly copy any field that is stored as a binary:

"path/to/csv/file"
|> File.stream!(read_ahead: 100_000)
|> MyParser.parse_stream
|> Stream.map(fn [name, age] ->
  %{name: :binary.copy(name),
    age: String.to_integer(age)}
end)

Dumping

NimbleCSV can dump any enumerable to either iodata or to streams:

iex> IO.iodata_to_binary MyParser.dump_to_iodata([~w(name age), ~w(mary 28)])
"name\tage\nmary\t28\n"

iex> MyParser.dump_to_stream([~w(name age), ~w(mary 28)])
#Stream<...>

Link to this section Summary

Functions

Defines a new parser/dumper

Callbacks

Eagerly dumps an enumerable into iodata (a list of binaries and bytes and other lists)

Lazily dumps from an enumerable to a stream

Same as parse_enumerable(enumerable, [])

Eagerly parses CSV from an enumerable and returns a list of rows

Same as parse_stream(enumerable, [])

Lazily parses CSV from a stream and returns a stream of rows

Same as parse_string(enumerable, [])

Eagerly parses CSV from a string and returns a list of rows

Link to this section Functions

Defines a new parser/dumper.

Calling this function defines a CSV module. Therefore, define is typically invoked at the top of your files and not inside functions. Placing it inside a function would cause the same module to be defined multiple times, one time per invocation, leading your code to emit warnings and slowing down execution.

It accepts the following options:

  • :moduledoc - the documentation for the generated module

The following options control parsing:

  • :escape- the CSV escape, defaults to "\""
  • :separator- the CSV separators, defaults to ",". It can be a string or a list of strings. If a list is given, the first entry is used for dumping (see below)
  • :newlines - the list of entries to be considered newlines when parsing, defaults to ["\r\n", "\n"] (note they are attempted in order, so the order matters)

The following options control dumping:

  • :escape- the CSV escape character, defaults to "\""
  • :separator- the CSV separator character, defaults to ","
  • :line_separator - the CSV line separator character, defaults to "\n"
  • :reserved - the list of characters to be escaped, it defaults to the :separator, :line_separator and :escape characters above.

Although parsing may support multiple newline delimiters, when dumping only one of them must be picked, which is controlled by the :line_separator option. This allows NimbleCSV to handle both "\r\n" and "\n" when parsing, but only the latter for dumping.

Parser/Dumper API

Modules defined with define/2 implement the NimbleCSV behaviour. See the callbacks for this behaviour for information on the generated functions and their documentation.

Link to this section Callbacks

Link to this callback

dump_to_iodata(rows) View Source
dump_to_iodata(rows :: Enumerable.t()) :: iodata()

Eagerly dumps an enumerable into iodata (a list of binaries and bytes and other lists).

Link to this callback

dump_to_stream(rows) View Source
dump_to_stream(rows :: Enumerable.t()) :: Enumerable.t()

Lazily dumps from an enumerable to a stream.

It returns a stream that emits each row as iodata.

Link to this callback

parse_enumerable(enum) View Source
parse_enumerable(enum :: Enumerable.t()) :: [[binary()]]

Same as parse_enumerable(enumerable, []).

Link to this callback

parse_enumerable(enum, opts) View Source
parse_enumerable(enum :: Enumerable.t(), opts :: keyword()) :: [[binary()]]

Eagerly parses CSV from an enumerable and returns a list of rows.

Options

  • :skip_headers - when true, skips headers. Defaults to true. Set it to false to keep headers or when the CSV has no headers.
Link to this callback

parse_stream(enum) View Source
parse_stream(enum :: Enumerable.t()) :: Enumerable.t()

Same as parse_stream(enumerable, []).

Link to this callback

parse_stream(enum, opts) View Source
parse_stream(enum :: Enumerable.t(), opts :: keyword()) :: Enumerable.t()

Lazily parses CSV from a stream and returns a stream of rows.

Options

  • :skip_headers - when true, skips headers. Defaults to true. Set it to false to keep headers or when the CSV has no headers.
Link to this callback

parse_string(binary) View Source
parse_string(binary()) :: [[binary()]]

Same as parse_string(enumerable, []).

Link to this callback

parse_string(binary, opts) View Source
parse_string(binary(), opts :: keyword()) :: [[binary()]]

Eagerly parses CSV from a string and returns a list of rows.

Options

  • :skip_headers - when true, skips headers. Defaults to true. Set it to false to keep headers or when the CSV has no headers.