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using System.Diagnostics.Contracts;


[ContractClass(typeof(ILanguageContract<>))]
public interface ILanguage<A>
{
    A Literal(string s);
    string Escape(string s);
    string Render(A a);
}

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Welcome to the inaugural Functional Friday post. I intend to explore, as far as is possible in C#, what functional programming brings to the table: types, laws, and equational reasoning. Example code is in the C# project at the root of this repository.

First up: parametricity and type classes.

Parametricity

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The previous post glossed over some features of C# that make functional programming difficult. Here they are explicitly addressed. Let’s call them warts. These warts make it difficult to reason about code and thus to write correct code.

Wart 1: null

Tony Hoare invented the null pointer and later apologised for doing so:

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Objects conflate construction, implementation hiding, (subtype) polymorphism, and the grouping of similar functionality. Algebraic Data Types (ADTs) help untangle these concepts.

In algebraic notation

Using the following notation:

• 1 for the unit type,

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Null pointer exceptions can be prevented at compile time using a simple data type borrowed from Haskell.

data Maybe a = Nothing | Just a

The Maybe type

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The type class encoding in my previous post on the subject is a bit rubbish:

• type class instances are classes, which have a non-zero creation cost;
• they do not have self-type parameters, so are not necessarily distinguishable at the type level; and
• they do not provide for derived operations.

The Maybe type I described could do with some type class instances, so let’s try out a new encoding.

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#!/bin/sh

PORT=9000
OUT_DIR='out'

while getopts ":f:p:d:" opt; do
  case $opt in
    f)
      HTML="$OPTARG"
      ;;

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#!/bin/sh

git diff-index --check --cached $against -- && \
  rm -rf out && \
  git stash save --keep-index 'pre-commit unstaged changes' && \
  lein clojurescript fresh test && \
  git stash pop

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function ngIt($injector) {
  return function(text, fn) {
    it(text, function(done) {
      var $rootScope = $injector.get('$rootScope')
        , fin
        , finished = function(err) {
            fin = true;
            done(err);
          };
      fn(finished);

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The brief

Create an executable (recapture) that plays a signal through 24 audio ports and simultaneously records from another 24 ports. These ports come from three daisy chained Presonus FP10’s connected by firewire to a linux box. This hardware is accessed by the ffado jack drivers.

The solution