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<| has wrong associativity and precedence
f#

The |> operator in F# has the right associativity (left) and precedence (low) to be quite useful. However, there are times when I really want something like the $ operator in Haskell, when the reversed argument ordering of |> is unnatural and I don't need that reversed ordering to help out type inference. On would think that you could just use the <| operator in this case, but it doesn't work out so well. If I want to make

myprint (run_attempt (fail_if_big 999))

more readable, I would expect to be able to just write

myprint <| run_attempt <| fail_if_big 999

but this doesn't work, because <| has the wrong associativity: it ought to be right associative, but it is left associative. Furthermore, according the the precedence table in the F# manual, it is also rather tightly binding, which is the opposite of what you would want for an operator such as this.

.

I have found one partial work-around: use of the << function composition operator. Thus one could write

myprint << run_attempt <| fail_if_big 999

This is only a partial workaround because << also binds rather

tightly, so you still have to add extra parentheses that are not needed

when using |> instead.

By on 3/18/2008 9:07 AM ()

Thanks for this input. We (the F# team) have registered this as a request for a design change for the associativity and precedence of <|, and we'll look at changing this in a future release.

Kind regards

don

By on 3/20/2008 1:21 AM ()

Hi,

It's sometimes useful to have left associativity for <| when the function has several arguments. I've written this kind of code a few times:

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printfn "%d, %d, %d"

  <| 3 * (2 + 4)

  <| 2 + 3

  <| abs (-4)

Or:

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max <| 3 * (5 - 2) <| f (3 * 2)

Current <| is useful to separate arguments and remove some parenthesis. But, I don't know which associativity is the best.

Laurent.

By on 3/20/2008 12:48 PM ()

Agreed.

I've been using the <| operator to clarify my code, but its left associativity has limited its usefulness. Its also rather anti-intuitive.

I assume the <OP %left precedence rule cannot be altered. Would inserting a low precedence <| %left rule be possible?

regards,

Daniel

By on 3/16/2008 9:28 AM ()

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