I shortly wrote about Type Providers for SQL databases and services in a blog post last year, and since then a range of new providers have been written by the community. Just a quick search and you can find providers for WMI, CSV, XML, Regular expressions, REST services, world bank and pretty much every other thing you can imagine. Especially FSharpX and some of the excellent work done by people like Tomas Petricek.
After looking around I was surprised to see some of the use-cases. I was thinking in the lines of what LINQ provides until I saw samples like the provider for regular expressions.
type PhoneRegex = Regex< @"(?<AreaCode>^\d{3})-(?<PhoneNumber>\d{3}-\d{4}$)">
PhoneRegex.IsMatch "425-123-2345"
|> should equal true
PhoneRegex().Match("425-123-2345").CompleteMatch.Value
|> should equal "425-123-2345"
PhoneRegex().Match("425-123-2345").PhoneNumber.Value
|> should equal "123-2345"
This made me think that if LINQ was a revolution that bridged the gap between SQL, XML and objects, then Type Providers looks to take consumption of data even further and help make any kind of data accessible in a very straight forward way. LINQ and Type Providers are however quite different technologies, that work on different levels.
Types as an abstraction
One of the things that looks promising about Type Providers is that implementing new providers seems to be easier than it is with LINQ - though it is still by no means trivial. The API for LINQ has been critiqued for being complex to implement, and it has even been called a broken abstraction. I love all the power LINQ provides and it is still a fair trade-off, but there is a point to be made about the implementations being very different from each other, and in conflict with principles like the Liskov Substitution Principle. Type Providers, as the name implies, work on the level of providing types rather than providing a common query language, so they don't have the same problem regarding abstractions. This also means that Type Providers are even more broadly applicable, since they can provide types from any source and for any kind of consumption including F# query expressions, which are similar to LINQ.
Working with schemaless data
Some of the first things I was wondering about when I first read about Type Providers was how they could work against e.g. CSV and REST services that don't have a fixed schema. It turns out that this is mostly solved with sample data, which is used to infer the schema information. To overcome cases where there is too much data some providers allow the user to supply a smaller sample or optionally specify the amount of data to read in before inferring the types.
type News = JsonProvider<"""[ { "text": "test1"}, { "text": "test2" } ]""">
let data =
Http.Request("http://api.somesite.com/newsservice",
headers=["content-type", "application/json"])
let res = News.Parse(data)
for item in res do
printfn " - %s" item.Text
Why is this especially interesting?
In todays world responsibilities are being moved to the client, and applications are built with Javascript frameworks like AngularJS or Knockout. This means servers are mainly providing data, but do so from a growing number of sources. With this in mind I think Type Providers make for a very valuable common use-case for F# in the web space.
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