F# Data: CSV Parser

The F# CSV Type Provider is built on top of an efficient CSV parser written in F#. There's also a simple API that can be used to access values dynamically.

When working with well-defined CSV documents, it is easier to use the type provider, but in a more dynamic scenario or when writing quick and simple scripts, the parser might be a simpler option.

Loading CSV documents

To load a sample CSV document, we first need to reference the FSharp.Data.dll library (when using F# Interactive) or to add reference to a project.

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#r "../../../bin/lib/net45/FSharp.Data.dll"
open FSharp.Data

The FSharp.Data namespace contains the CsvFile type that provides two static methods for loading data. The Parse method can be used if we have the data in a string value. The Load method allows reading the data from a file or from a web resource (and there's also an asynchronous AsyncLoad version). The following sample calls Load with a URL that points to a live CSV file on the Yahoo finance web site:

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// Download the stock prices
let msft = CsvFile.Load("http://www.google.com/finance/historical?q=MSFT&output=csv").Cache()

// Print the prices in the HLOC format
for row in msft.Rows do
  printfn "HLOC: (%s, %s, %s)" 
    (row.GetColumn "High") (row.GetColumn "Low") (row.GetColumn "Date")

Note that unlike CsvProvider, CsvFile works in streaming mode for performance reasons, which means that Rows can only be iterated once. If you need to iterate multiple times, use the Cache method, but please note that this will increase memory usage and should not be used in large datasets.

Using CSV extensions

Now we look at a number of extensions that become available after opening the FSharp.Data.CsvExtensions namespace. Once opened, we can write:

• row?column uses the dynamic operator to obtain the column value named column; alternatively, you can also use an indexer row.[column].
• value.AsBoolean() returns the value as boolean if it is either true or false
• value.AsInteger() returns the value as integer if it is numeric and can be converted to an integer; value.AsInteger64(), value.AsDecimal() and value.AsFloat() behave similarly.
• value.AsDateTime() returns the value as a DateTime value using either the ISO 8601 format, or using the \/Date(...)\/ JSON format containing number of milliseconds since 1/1/1970.
• value.AsGuid() returns the value as a Guid value.

Methods that may need to parse a numeric value or date (such as AsFloat and AsDateTime) receive an optional culture parameter.

The following example shows how to process the sample previous CSV sample using these extensions:

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open FSharp.Data.CsvExtensions

for row in msft.Rows do
  printfn "HLOC: (%f, %M, %O)" 
    (row.["High"].AsFloat()) (row?Low.AsDecimal()) (row?Date.AsDateTime())

Transforming CSV files

In addition to reading, CsvFiles also has support for transforming CSV files. The operations available are Filter, Take, TakeWhile, Skip, SkipWhile, and Truncate. After transforming you can save the results by using one of the overloads of the Save method. You can choose different separator and quote characters when saving.

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// Saving the first 10 stock prices where the closing price is higher than the opening price in TSV format:
msft.Filter(fun row -> row?Close.AsFloat() > row?Open.AsFloat())
    .Truncate(10)
    .SaveToString('\t')

Related articles

• F# Data: CSV Type Provider - discusses F# type provider that provides type-safe access to CSV data
• API Reference: CsvFile class
• API Reference: CsvRow class
• API Reference: CsvExtensions module

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