Skip to main content

C# for Haskell and ML Programmers

An interesting question was posed to /r/Haskell today: is there a quick intro to C# from Haskell programmers?

Well there are plenty of explanations of C#'s basic syntax, classes, structs, etc., but nothing that specifically addresses the functional mindset a Haskell programmer would be starting with, so I wrote a reply providing links to the various familiar concepts from functional programming found in C#, and described various caveats that might be surprising to a Haskell user. I'll reproduce the post here for posterity:

I think there's a reasonable C# subset for functional programming, so if you stick to that you should be able to pick it up relatively quickly. Read up on:

  • LINQ -- you can use the query comprehension syntax, or the regular first-class function syntax. The former is sugar for the latter.
  • Tuples
  • Lambdas and delegates (delegates are first-class functions)
  • Parametric polymorphism is known as generics. You can place generic parameters on methods/functions, and type declarations.
    • The standard System.Func* and System.Action* delegates. These are delegates with generic parameters. Most first-class functions you deal with will be one of these two classes of delegates.
  • Type constraints

Caveats:

  • lambdas and delegates are nominally typed, so you can't implicitly or explicitly coerce a delegate of one type into a delegate of a compatible signature, ie. System.Predicate<int> is signature compatible with System.Func<int, bool>, but they are not interconvertible without doing some magic like I do in my Sasa.Func.Coerce library function.
  • methods and delegates are multiparameter, not curried like in OCaml and Haskell, thus leading to all the Func* and Action* overloads.
  • The "void" return type is not a type, so it can't be used as a generic argument. Hence the need for all the Action* delegate types distinct from the Func* delegate types. Action* differ only in the fact that they return void.
  • generic parameters on methods are strictly more general than generic parameters on delegates (which are types). Method generics support first-class polymorphism, while type declaration generics do not.
  • classes are always implicitly option types, ie. they are nullable, while struct types always have a "valid" value, ie. are not nullable. Struct types are then useful for eliminating null reference exceptions in programs, as long the default struct value is meaningful.
Labels:

Comments

Post a Comment

Popular posts from this blog

Blue-eyed Islander Puzzle - an analysis

Many people find themselves stumped by the so-called Blue-Eyed Islanders puzzle . There is also much controversy over its supposed solution. I'm going to analyze the problem and the solution, and in the process, explain why the solution works. To begin, let's modify the problem slightly and say that there's only 1 blue-eyed islander. When the foreigner makes his pronouncement, the blue-eyed islander looks around and sees no other blue eyes, and being logical, correctly deduces that his own eyes must be blue in order for the foreigner's statement to make sense. The lone blue-eyed islander thus commits suicide the following day at noon. Now comes the tricky part, and the source of much confusion. Let's say there are 2 blue-eyed islanders, Mort and Bob. When the foreigner makes his pronouncement, Mort and Bob look around and see only each other. Mort and Bob thus both temporarily assume that the other will commit suicide the following day at noon. Imagine their chagrin...
40 comments
Read more

Easy Reverse Mode Automatic Differentiation in C#

Continuing from my last post on implementing forward-mode automatic differentiation (AD) using C# operator overloading , this is just a quick follow-up showing how easy reverse mode is to achieve, and why it's important. Why Reverse Mode Automatic Differentiation? As explained in the last post, the vector representation of forward-mode AD can compute the derivatives of all parameter simultaneously, but it does so with considerable space cost: each operation creates a vector computing the derivative of each parameter. So N parameters with M operations would allocation O(N*M) space. It turns out, this is unnecessary! Reverse mode AD allocates only O(N+M) space to compute the derivatives of N parameters across M operations. In general, forward mode AD is best suited to differentiating functions of type: R → R N That is, functions of 1 parameter that compute multiple outputs. Reverse mode AD is suited to the dual scenario: R N → R That is, functions of many parameters t...
18 comments
Read more

Extensible, Statically Typed Pratt Parser in C#

I just completed a statically typed Pratt-style single-state extensible lexer+parser, otherwise known as a top-down operator precedence parser, for the Sasa library . The implementation is available in the Sasa.Parsing dll, under Sasa.Parsing.Pratt . Two simple arithmetic calculators are available in the unit tests . This implementation is novel in two ways: Aside from an alleged implementation in Ada , this is the only statically typed Pratt parser I'm aware of. Pratt parsers typically require a pre-tokenized input before parsing semantic tokens, but I've eliminated this step by using the symbol definitions to drive a longest-match priority-based lexer. Symbols by default match themselves, but you can optionally provide a scanning function used to match arbitrary string patterns. The symbol selected for the current position of the input is the symbol that matches the longest substring. If two symbols match equally, then the symbol with higher precedence is selected. The design...
12 comments
Read more