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I understand the reasons why I would prefer let a = f(x) over let a() = f(x), especially when f takes is a long running function.

I also think it is correct to say, that, considering lambda calculus origins of functional programming let a() is truly functional, while let a is not. Please correct me if I'm wrong on this.

However I have no clue which expression will actually be better:

let a = 4


let a() = 4

Is there any reason I would prefer one over the other?

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If anything, 4 is not "functional" (in the sense of being native to lambda calculus). let x = foo in bar is spelled (\x bar) (foo) in lambda calculus, and let x() = foo in bar is, if I understand F# correctly, just (\x -> bar) (\_ foo). – delnan Aug 29 '14 at 16:06
up vote 3 down vote accepted

There's nothing "unfunctional" about let a = 4 as opposed to let a () = 4. In fact, in many abstract formulation of type theories, the latter is really syntactic sugar for let a = fun () -> 4.

The main difference is when you want the expression to be evaluated, as you said. If a will eventually be evaluated anyway, there's no point in wrapping it into a function. OTOH if you think that there's a good chance that a might not be evaluated, and the evaluation is very expensive (or may even fail if some precondition is not satisfied), then it might be beneficial to keep as a function. However, in the latter case you still have to be careful to not evaluate the function more than once.

However, I'm not sure if pervasive use of laziness is all that common in a strict language such as F#.

Edit to address your question in the comments:

In many formal mathematical specifications of a language, the let construct binds an expression to a single variable:


The reason is because this is simpler than having a let that binds arbitrary functions such as let f x = 2 * x. Instead, the latter is treated as a syntactic sugar for the more verbose let f = fun x -> 2 * x.

However, neither is more "functional" (if that word even has a well-defined meaning). Functional programming does not mean you should attempt to use functions everywhere possible. Rather, it merely puts functions on equal standing with other values so that you can manipulate both functions and values with equal ease.

The let construct in Haskell looks superficially like the one in F#, but semantically it is very different because it's non-strict ("lazy") by default. This means let x = 4 in 2 * x in Haskell is more appropriately translated into let x = lazy(4) in 2 * Lazy.force x.

The let construct should not be confused with assignments, which are an entirely different thing as it requires mutation. By constrast, let does not mutate anything: it merely introduces a new variable. F# blurs the boundary because it allows pure and impure functions to be mixed, so let bindings can have side-effects as a result and hence the way you bind them (as functions or values) will matter.

In a pure language, let bindings cannot cause any side-effects, but the order of evaluation still matters for two reasons:

  • Efficiency: because we don't have infinite resources and infinite patience.
  • Divergence: functions can still fail, or go into an infinite loop.

So even in pure languages, it still matters whether evaluation is done eagerly or lazily.

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IF you wanted, you could implement memoization to get the best of both worlds. – Christopher Stevenson Aug 29 '14 at 17:10
Could You please explain there's nothing "unfunctional" in let a = 4? And why You say that let a() = 4 is opposite to that (so if I understood You well, has something "unfunctional")? Is that because it is not a lambda but just a regular function. On the other hand, f# is not purely functional language for sure. Is similar assignment (`let a = 4) possible in Haskell? – Grzegorz Sławecki Aug 29 '14 at 18:55
Added some explanation. Hope that addresses your questions! – Rufflewind Aug 29 '14 at 19:16

Apart from lambda calculus related considerations, there are as well practical considerations. Whether you create a value (let a = 4) or a function value (let a() = 4) has an impact on when your code gets executed, which, at times is of importance. The following F# code shows the effect in a simple way:

let A = printfn "A invoked"
let B() = printfn "B invoked"

let Run() =
    printfn "Run() invoked"


The output is:

A invoked
Run() invoked
B invoked

This might come as a surprise once in a while. For simple constant numerical values, less so, than for some functions producing side effects (printfn,...).

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Don't be concerned about which is more "functional." You're a programmer, not a mathematician. Use let a() when you want to evaluate it at the point of use, when you don't care about evaluating it multiple times. Use let a when you want to evaluate it at the definition point, and only want it to be evaluated once.

Your third choice is let a = lazy(f(x)), which waits until it is needed to evaluate, but only evaluates it once. This is my preferred choice for expensive functions that may not be needed. This is kind of ugly in F# though, because it requires an explicit a.Force, unlike most functional languages which force evaluation automatically at the appropriate time.

Remember, let a looks sort of like a mutating assignment, but it isn't. Pure functional programs can be substituted and expanded out to one big nested expression. Conceptually, if you expand your program out, you still end up with essentially the same expression with let a, it just eliminates some duplication when you evaluate it.

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I guess we could have a discussion about "You're a programmer, not a mathematician." someday, somwhere :-) Anyway, thank You for Your point. – Grzegorz Sławecki Aug 30 '14 at 8:30

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