TP

Comparing date range handling in C# and F#

I was recently working on some code for handling date ranges in Deedle. Although Deedle is written in F#, I also wrote some internal integration code in C#. After doing that, I realized that the code I wrote is actually reusable and should be a part of Deedle itself and so I went through the process of rewriting a simple function from (fairly functional) C# to F#. This is a small (and by no means representative!) example, but I think it nicely shows some of the reasons why I like F#, so I thought I'd share it.

One thing that we are adding to Deedle is a "BigDeedle" implementation of internal data structures. The idea is that you can load very big frames and series without actually loading all data into memory.

When you perform slicing on a large series and then merge some of the parts of the series (say, years 2010, 2012 and 2014), you end up with a series that combines a couple of chunks. If you then restrict the series (say, from June 2012 to June 2014), you need to restrict the ranges of the chunks:

Demonstration

As the diagram shows, this is just a matter of iterating over the chunks, keeping those in the range, dropping those outside of the range and restrictingthe boundaries of the other chunks. So, let's start with the C# version I wrote.

Published: Wednesday, 22 April 2015, 4:55 PM
Tags: f#, c#, deedle, linq, functional programming
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Writing custom F# LINQ query builder

One of the attendees of my virtual F# in Finance course, Stuart recently asked me a pretty advanced question about writing custom queries with F#, because he was interested in writing a nicer querying library for Amazon DynamoDB (his project is here).

The DynamoDB could even be a type generated by a type provider (with all the tables available in Dynamo DB). Now, the above example uses the built-in query builder, which is extensible, but, as far as I know, you have to use LINQ expression trees to support it. In this article, I'm going to use an alternative approach with custom builder (so you would write dynamo { ... } instead of query { ... }).

I wanted to write a minimal example showing how to do this, so this blog post is going to be mostly code (unlike my other chatty articles!), but it should give you (and Stuart :-)) some idea how to do this. I was quite intrigued by the idea of having a nice query language for DynamoDB, so I'm hoping that this blog post can help move the project forward!

namespace Microsoft
namespace Microsoft.FSharp
namespace Microsoft.FSharp.Quotations
module Patterns

from Microsoft.FSharp.Quotations
module DerivedPatterns

from Microsoft.FSharp.Quotations
type People =
  {Name: string;
   Age: int;}

Full name: Query-translation.People
People.Name: string
Multiple items
val string : value:'T -> string

Full name: Microsoft.FSharp.Core.Operators.string

--------------------
type string = System.String

Full name: Microsoft.FSharp.Core.string
People.Age: int
Multiple items
val int : value:'T -> int (requires member op_Explicit)

Full name: Microsoft.FSharp.Core.Operators.int

--------------------
type int = int32

Full name: Microsoft.FSharp.Core.int

--------------------
type int<'Measure> = int

Full name: Microsoft.FSharp.Core.int<_>
type DynamoDB =
  static member People : seq<People>

Full name: Query-translation.DynamoDB
Multiple items
static member DynamoDB.People : seq<People>

Full name: Query-translation.DynamoDB.People

--------------------
type People =
  {Name: string;
   Age: int;}

Full name: Query-translation.People
Multiple items
val seq : sequence:seq<'T> -> seq<'T>

Full name: Microsoft.FSharp.Core.Operators.seq

--------------------
type seq<'T> = System.Collections.Generic.IEnumerable<'T>

Full name: Microsoft.FSharp.Collections.seq<_>
module Seq

from Microsoft.FSharp.Collections
val empty<'T> : seq<'T>

Full name: Microsoft.FSharp.Collections.Seq.empty
val query : Linq.QueryBuilder

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.query
val p : People
property DynamoDB.People: seq<People>
custom operation: where (bool)

Calls Linq.QueryBuilder.Where
custom operation: select ('Result)

Calls Linq.QueryBuilder.Select

Published: Tuesday, 7 April 2015, 1:41 PM
Tags: f#, functional programming, linq
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Pattern matching in action using C# 6

On year ago, on this very day, I wrote about the open-sourcing of C# 6.0. Thanks to a very special information leak, I learned about this about a week before Microsoft officially announced it. However, my information were slightly incorrect - rather then releasing the much improved version of the language, Microsoft continued working on language version internally called "Small C#", which is now available as "C# 6" in the Visual Studio 2015 preview.

It is my understanding that, with this release, Microsoft is secretly testing the reaction of the developer audience to some of the amazing features that F# developers loved and used for the last 7 years and that are coming to C# very soon. To avoid shock, these are however carefuly hidden!

In this blog post, I'm going to show you pattern matching which is probably the most useful hidden C# feature and its improvements in C# 6. For reasons that elude me, pattern matching in C# 6 is called exception filters and has some unfortunate restrictions. But we can still use it to write nice functional code!

Published: Wednesday, 1 April 2015, 11:41 AM
Tags: c#, fun, functional programming
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