From Fixed Points to Recursion

Recursion refers to self-referential code. Most people are familiar with recursion in the form of names that are used before their values are fully computed. The classic Fibonacci function can be used to illustrate this. As you can see, the definition of fib references itself.

fib :: Int -> Int
fib 0 = 0
fib 1 = 1
fib n = fib (n - 1) + fib (n - 2)

The fix function in Haskell calculates the least fixed point of the function provided as argument, if it exists. From numerical analysis, you may recall a fixed point as a value at which the output remains unaltered no matter how many times you apply the function to it. In other words, the following statement is true for all f, for which the fix function can be computed:

f (fix f) = fix f

To use the fix function, we first need to a redefine fib as a function that can be supplied to it. We define a new variable f that represents the Fibonacci function, moving fib over to the right side.

let f = \fib -> \n -> if n < 2 then n else fib (n - 1) + fib (n - 2)

In plain English, you could read this as: given the Fibonacci function and a number, we can calculate the value as (a) the number itself if it is less than 2, or (b) the sum of the Fibonacci function applied to the previous two numbers respectively.

Notice that the definition above no longer uses recursion; it simply accepts fib and n as arguments, and calculates the result.

We now find the least fixed point for the Fibonacci function, and apply it to the desired input. For instance, to find the 11th Fibonacci number, we write:

import Data.Function (fix)
fix f 11

…and voila! it prints the result 89.

If you open up the source code of fix, this is how it is defined:

fix :: (a -> a) -> a
fix f = let x = f x in x

Again, in plain English, replace x with the supplied function f applied to (f applied to (f applied to (…))), then return x. You would think this would go into an infinite loop — and it does, if the function doesn’t converge — but it actually works! One of the advantages of a language with non-strict evaluation semantics like Haskell is the ability to work effectively with infinite regress.

Link to GitHub

That鈥檚 all for today, folks! 馃枛

The Command Pattern

Modularity is a desirable property of any reasonably complex software. Modularity allows the programmer to examine, understand and change parts — modules — of the software while temporarily ignoring the rest of it. When the software becomes too large for a single programmer to work on it sequentially, modularity allows a team of individuals to work on it in parallel. Ideally, modularity is recursive — modules should themselves consist of modules, and so on, until each module is small enough for an individual to grasp quickly, even with an arbitrarily large team.

From one perspective, modularity is less about breaking down software into smaller modules, and more about creating small modules that can be easily combined with other modules to create large systems. Combining modules is called ‘composition’, and composability is the holy grail of software design.

Functions are the ultimate tool in the toolbox of composition. In mathematics, a function has inputs and outputs, and its definition represents the mapping of inputs to outputs. In the computational world, these mappings may be viewed as transformations of inputs into outputs. Functions are inherently composable, as under the right conditions, the outputs of one function may be connected to the input of another function (even itself). Unfortunately, functions in mainstream programming languages are impure in the sense that they may do other things, such as write bytes to disk or send data over a network. These so-called ‘effects’ hinder our ability to compose functions using their mathematical representations, unless the effects themselves are modeled as first-class inputs or outputs.

Only a few languages like Haskell support pure functions — functions that are free of effects. In Haskell, effects are possible only if they are modeled as first-class inputs or outputs. In these cases, effects are encoded into the runtime instances of a special type called IO, and it is the responsibility of language runtime to execute these effects on behalf of the programmer. For example, in the program below, the main function returns an IO instance, and the language runtime executes the effects encapsulated by the instance. In fact, without resorting to backdoor (aka unsafe) techniques, it is not possible to specify effects within a function that doesn’t return an IO instance. With the IO type, Haskell programs are smart enough to declare which functions are effectful, and these functions can be distinguished from ones that are not.

import System.IO (BufferMode (NoBuffering), hSetBuffering, stdout)

-- Main program.
-- This function returns an IO instance.
main :: IO ()
main = do
    hSetBuffering stdout NoBuffering          -- :: IO ()
    putStr $ "Enter a number x: "             -- :: IO ()
    x <- getLine                              -- :: IO String
    putStr $ "Enter a number y: "             -- :: IO ()
    y <- getLine                              -- :: IO String
    putStrLn $ show $ mult (read x) (read y)  -- :: IO ()

-- Multiply two numbers.
-- This function cannot write to disk or send data over the network.
mult :: Int -> Int -> Int
mult x y = x * y

Effects represented by IO instances can themselves be combined, but only sequentially. In the example above, each line within the main function returns some kind of IO instance. These IO instances are strung together to create a single combined expression…which is itself an IO instance. Furthermore, the computational results of an IO instance can never be extracted into a pure function: once you enter the real world, you can never come back.

Object Oriented Languages

In object-oriented languages like Java, composability still remains the holy grail of software development, and so lessons from the functional world are applicable. The essence of the ideas described above can be boiled down into a simple rule of thumb —

When you need to perform an action that deals with the external world, like writing to disk or sending data over the network, encapsulate the action within a ‘command’, and separate the decision of performing the action from actually performing the action.

This separation allows you as the programmer to inspect, re-arrange and re-compose your effectful code easily. Given adequately precise shapes for commands, the compiler will even aid you in making these changes safe. The command interface is analogous to the IO type in Haskell. And just like IO, you can string together commands to construct more sophisticated composite ones.

Once you are speaking the language of commands, you can perform computations on the commands themselves. For instance, suppose that instead of printing a message to the screen, you create a ‘log statement’ object that is capable of printing a message to the screen, and then invoke it. You can now enrich all log statements with timestamps, apply filtering based on various criteria and perform other actions before or after you print messages. As another example, suppose that instead of calling a remote web service, you create a ‘service invoker’ object that is given all of the information it needs to call the remote web service, and then invoke it. You can now apply throttling and caching mechanisms that control the flow of how these effects are performed.

The real world is messy, uncertain and error-prone. If we can limit the interactions of our systems with the external world to a few points at the edges, our software is that much more robust, and easier to develop, operate and maintain.

That鈥檚 all for today, folks! 馃枛

The Untyped 位-Calculus

The untyped 位-calculus is a formal system in mathematical logic that expresses computations. Each computation is presented as a term, and terms may themselves be composed of additional terms. Every term is either a variable, an abstraction or an application. Evaluating a program is equivalent to starting with a term and performing a series of substitutions, based on a set of formal rules.


The ABNF grammar for the untyped 位-calculus is shown below. For simplicity, we assume that whitespace is not significant except as a delimiter between terms (where needed).

term := var | abs | app
abs  := "位" var "." term
app  := term term


Given below is an example of a well-formed term, where x, y and z are variables, \lambda x.x and \lambda are abstractions, xyz and the term as a whole are applications. Here, we take the liberty of assuming that xy actually denotes two separate variables (the application of x to y).

(\lambda x.x)(\lambda

Note that there are actually four variables in this expression – the x variables in the two parenthesized terms are completely unrelated as they are bound to different abstractions. When a variable is not bound to any abstraction, it is said to be free.

\left(\overbrace{\lambda x.\underbrace{x}_{\text{\footnotesize Bound Variable}}}^{\text{\footnotesize Abstraction}}\right)\left(\overbrace{\lambda xy.xy\underbrace{z}_{\text{\footnotesize Free Variable}}}^{\text{\footnotesize Abstraction}}\right)

Substitution Rules


Within the scope of an abstraction, a bound variable may be freely substituted with any symbol that isn’t already in use (which means it is neither free nor bound within the current context). This is known as an 伪-conversion. For instance, the example above is equivalent to the following expressions.

\left(\lambda x.x\right)\left(\lambda wk.wkz\right)
\left(\lambda y.y\right)\left(\lambda\right)

However, it is not equivalent the following expression, as it inadvertently converts the free z into a bound one.

\left(\lambda x.x\right)\left(\lambda zy.zyz\right)


When an abstraction is applied to a term, the former can be reduced by substituting every occurrence of the first bound variable of the abstraction with the term it is applied to. This is known as a 尾-reduction. In traditional programming languages, this corresponds to function application, converting a complex expression into something simpler. Our original example above reduces to the following.

\left(\lambda x.x\right)\left(\lambda wk.wkz\right)\xrightarrow{尾}\lambda wk.wkz

The first term, (\lambda x.x), is equivalent to a function that returns its argument as-is – the identity function. Applying the identity function to the second term simply returns the second term.

Note that 尾-reduction doesn’t always simplify the term. In some cases, further reduction may yield the same term ad infinitum, in which case the term is said to be in a ‘minimal form’. In other cases, the term may actually become bigger with each reduction, in which case it is said to diverge. In all other cases, when no further 尾reduction is possible, the term is said to be in 尾-normal form.

\left(\lambda x.x x\right)\left(\lambda x.x x\right) \tag{\text{Minimal}}
\left(\lambda x.xxy\right)\left(\lambda x.xxy\right) \tag{\text{Divergent}}

Writing an interpreter for the untyped 位-calculus is relatively straightforward in Haskell.


  • Make sure the stack is installed on your system.
  • Clone the package from GitHub to run the code.
$ git clone
$ cd untype
$ stack build
$ stack test
$ stack exec -- untype

A Quick Walkthrough

Link to GitHub

The core data structure to represent terms mimics the ABNF grammar described earlier. This recursive type declaration is easy to define in Haskell, as it uses a non-strict evaluation strategy.

data Term
    = Var Sym       -- ^ Variable
    | Abs Sym Term  -- ^ Abstraction
    | App Term Term -- ^ Application
    deriving (Eq)

For contrast, a similar data structure in Rust would have looked like this (notice the Box type that adds a level of indirection).

pub enum Term {
    Abs(Sym, Box<Term>),
    App(Box<Term>, Box<Term>),

The general strategy here is to accept an expression as newline-terminated text, apply a parser to the input to derive an abstract syntax tree, apply 伪-conversion and 尾-reduction strategies on the term until it converges, and then finally print it to the screen.

We use attoparsec, a nifty parser-combinator library, to write simple parsers and combine them into larger ones. As we apply a few basic parsers recursively, we need to look out for a few gotchas, called out in the code, that might cause the parser to loop indefinitely or give us incorrect results.

Finally, determining when to generate fresh variables and how to name them is surprisingly challenging. Here is an example when variable names must be substituted prior to reduction.

\overbrace{\left(\lambda x y.x y y\right)}^{\text{\footnotesize Term 1}}\overbrace{\left(\lambda u.u y x\right)}^{\text{\footnotesize Term 2}}

Here, Term 2 needs to replace the x within Term 1 as part of the 尾-reduction step. However, Term 2 already has a y, conflicting with the y – a distinct bound variable – in Term 1. We therefore need to replace the y in Term 1 with a fresh variable, and only then proceed with the substitution.

We leverage a simple strategy for generating fresh variables. First, we collect free variables across the whole term. Then, as we traverse the term, we keep track of all bound variables in the current context. Whenever we need a fresh variable, we take the original and append an apostrophe (') at the end. We then check this new variable against the list of free variables as well as the list of bound variables in the current context. If there are no collisions, we’re done; if not, we repeat the process.

That鈥檚 all for today, folks! 馃枛