Charlie Harvey

Quick tip: Postfix operators in GHCi

The other day I wanted to work out what 52 factorial was. I’d beeen to a pub quiz the night before. What can I say?

It is pretty easy to make the factorial function in Haskell. Something ike this usually suffices. fac :: Integer -> Integer fac n = product [1..n]

I wanted to be a bit clever and make a proper postfix operator, so that I could write 52! As it turns out there does exist a Postfix operator GHCi extension. It is described thus

The -XPostfixOperators flag enables a small extension to the syntax of left operator sections, which allows you to define postfix operators. The extension is this: the left section

(e !)
is equivalent (from the point of view of both type checking and execution) to the expression
((!) e)
(for any expression e and operator (!). The strict Haskell 98 interpretation is that the section is equivalent to
(\y -> (!) e y)
That is, the operator must be a function of two arguments. GHC allows it to take only one argument, and that in turn allows you to write the function postfix.

It took me a few minutes to work out how to get the following code to run in GHCi.{-# LANGUAGE PostfixOperators #-} (!) :: Integer -> Integer (!) n = product [1..n]

As it turns out the solution was rather straightforward. I needed to load the extension directly in GHCi thus :set -XPostfixOperators

So now I can write 52 factorial almost properly, though I still need to use brackets. *Main> :l fact.hs [1 of 1] Compiling Main ( fact.hs, interpreted ) Ok, modules loaded: Main. *Main> :set -XPostfixOperators *Main> (52!) 80658175170943878571660636856403766975289505440883277824000000000000 *Main>


  • Be respectful. You may want to read the comment guidelines before posting.
  • You can use Markdown syntax to format your comments. You can only use level 5 and 6 headings.
  • You can add class="your language" to code blocks to help highlight.js highlight them correctly.

Privacy note: This form will forward your IP address, user agent and referrer to the Akismet, StopForumSpam and Botscout spam filtering services. I don’t log these details. Those services will. I do log everything you type into the form. Full privacy statement.