Partial Application in F#

Having done a small amount of streaming with F# and seen how people react to partial application of functions, especially those who are already programmers in another language like C# or Java - I thought it might be prudent to have a look at partial application in F# and some of the uses of it.

Let's say that we have the following function definition:

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let add a b = a + b

Just a simple function that takes two integers and adds them thus:

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let simpleAdd = add 4 5

To give the variable simpleAdd a value of:

9

That looks like a function that takes 2 arguments, however we must remember that F# curries functions by default and therefore the above definition is equivalent to the following:

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let add' a =
    let add'' b =
        a + b
    add''

Hang on - what's that word "Currying" all about? Well it's named after Haskell Curry as is the programming language of Haskell.

That didn't help much did it?

Fair enough - Currying is where a language treats all functions as functions that take a single argument and return a value. There is nothing to say that the value returned can't be another function:

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let add2 = add 2
let add2' = add' 2

Above are invocations of the add and add' functions. Hover over the add2 and add2' - go on... I'm not going anywhere. What did you notice? They are of the same type aren't they? Both int -> int if I'm not much mistaken. Excellent - now if you're surprised by this then I guess you were expecting an error to the effect of I'm terribly sorry but you haven't given me enough information to run - or at least that might be the case for add2

What happens instead looks like some form of suspended animation if you like, where add2 waits for the other argument before it kicks off and runs. In reality we're working with one-parameter (or one-argument) functions. Interestingly, to me anyway, this does seem to explain the rather odd notation for a function signature in F#, being in this case int -> int -> int. There is no distinction between successive parameters and the return type as F# is fine with you treating it as a function that takes an int and returns a function of type int -> int or one that takes two ints in succession to return a single int (I tend to read -> as "to" or "goes to", so int -> int -> int is a function of int to int to int.)

Use in the real world.

Let's say we have a function:

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let doesSomething x =
    printfn "Doing something"
    x + 1

All we're saying here is that we want to do something with x but we want to log it to the console first. That's all fine and works nicely but someday your co-worker or boss really wants that logging to go to a file in some cases and to the console in others. Now you could write functions to do both. Let's not though, we're not interested in that kind of duplication. How about this instead?

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let doesSomethingWithALogger logger x =
    logger "Doing something"
    x + 1

let doesSomethingConsole = doesSomethingWithALogger (printfn "%s")

let doesSomethingWithNoLogging = doesSomethingWithALogger ignore

We've just created a function that takes in a logger and then two functions that only pass in loggers. doesSomethingConsole, doesSomethingWithNoLogging and doesSomething are all of type int -> int. They're all drop-in replacements for each other. The eagle-eyed among you might have spotted that we've used partial application another time: printfn "%s" - that's because printfn doesn't take a string, it takes some Printf.TextWriterFormat with a generic type, that's another post though.

Hopefully you now understand the idea behind partial application, how currying enables it and how injecting behaviour can clean up your codebase significantly.