F# tail call broken by lack of parentheses

Question

While testing F# tail calls with F# team blog article I found that almost the same code has the same result but different IL although only parentheses in the code differ.

Next code is optimized by compiler and I see br.s IL_0000 in the end of IL and no calls to sumSoFar

let rec loopAndSum aList sumSoFar = 
    match aList with
    | [] -> sumSoFar
    | x :: xs -> 
        loopAndSum xs (sumSoFar + x)

loopAndSum [ 1..5 ] 0 
|> printfn "sum: %i"

But that piece is not optimized by compiler and it has call bla_bla.loopAndSum near to the end of IL.

let rec loopAndSum aList sumSoFar = 
    match aList with
    | [] -> sumSoFar
    | x :: xs -> 
        loopAndSum xs sumSoFar + x

loopAndSum [ 1..5 ] 0 
|> printfn "sum: %i"

These examples differ only with parentheses around sumSoFar + x. You can play with it and look at IL at .NET Fiddle.

Does anybody know why parentheses matter?

Did you actually test to see if they did the same thing. The second version will always have `sumSoFar` set to 0. — John Palmer, Mar 07 '15 at 21:05
@JohnPalmer, both versions print `sum: 15` locally in LinqPad and in the fiddle. — Roman Dibikhin, Mar 08 '15 at 08:43
if you print `sumSoFar` each time you will get different results. — John Palmer, Mar 08 '15 at 08:44
@JohnPalmer, I found that not. Compose example at fiddle, please. — Roman Dibikhin, Mar 08 '15 at 08:57
@JohnPalmer, yeah, I found it :) `sumSoFar` is extra parameter here. It can be removed and replaced w/ zero in `match`. It's a more trickier example than I thought. Thank you ;) — Roman Dibikhin, Mar 08 '15 at 09:43

score 4 · Accepted Answer · answered Mar 07 '15 at 20:08

4

Function application has higher precedence than any operator. So without the parentheses, it is equivalent to:

(loopAndSum xs sumSoFar) + x

Therefore it is not a tail call: the addition is performed after the recursive call. It is just by chance that the result value is correct.

answered Mar 07 '15 at 20:08

Tarmil

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thank you for clarification. Could you point at spec where it's described? – Roman Dibikhin Mar 08 '15 at 08:55
At the bottom of [this page](https://msdn.microsoft.com/en-us/library/dd233228.aspx) there is an operator precedence table, function application is near the bottom between `**` and pattern matching. – Tarmil Mar 08 '15 at 11:40

F# tail call broken by lack of parentheses

1 Answers1