Haskell has immutable variables (variables in the math sense) by default: foo x y = x + y * 2 By default variables are not mutable cells. Haskell also has mutable cells though, but you enable them explicitly: > import Data.IORef (newIORef, readIORef, writeIORef) > v <- newIORef 0 > readIORef v 0 > writeIORef … Read more
The new Racket language (formerly PLT Scheme) allows you to implement any semantics you like with s-expressions (really any syntax). The base language is an eagerly evaluated, dynamically typed scheme variant but some notable languages built on top are a lazy scheme and a functional reactive system called Father Time. An easy way to make … Read more
Take the following mini-language: data Action = Get (Char -> Action) | Put Char Action | End Get f means: read a character c, and perform action f c. Put c a means: write character c, and perform action a. Here’s a program that prints “xy”, then asks for two letters and prints them in … Read more
The Languages We Call Haskell unsafePerformIO is part of the Foreign Function Interface specification, not core Haskell 98 specification. It can be used to do local side effects that don’t escape some scope, in order to expose a purely functional interface. That is, we use it to hide effects when the type checker can’t do … Read more
If I gather in one place any three theorists of my acquaintance, at least two of them disagree on the meaning of the term “referential transparency.” And when I was a young student, a mentor of mine gave me a paper explaining that even if you consider only the professional literature, the phrase “referentially transparent” … Read more
The term “referential transparency” comes from analytical philosophy, the branch of philosophy that analyzes natural language constructs, statements and arguments based on the methods of logic and mathematics. In other words, it is the closest subject outside computer science to what we call programming language semantics. The philosopher Willard Quine was responsible for initiating the … Read more