user2051561 - 1 year ago 71

Scala Question

I have a type inference issue and asked for help here.

The initial problem was due to overload. Once corrected I

still had problems.

So here is the code:

`class DPipe[ A ]( a: A ) {`

def !>[ B ]( f: A => B ) = Try(f( a ))

def #>[ B, C ]( f: B => C )(implicit ev: A =:= Try[B]) : Try[C] = a.map(f)

//def &>[ B, C ]( f: B => C )( implicit ev: A =:= Try[ B ] ) = a.map( f )

}

object DPipe {

def apply[ A ]( v: A ) = new DPipe( v )

}

object DPipeOps {

implicit def toDPipe[ A ]( a: A ): DPipe[ A ] = DPipe( a )

}

And here are the tests:

`object DPipeDebug {`

def main( args: Array[ String ] ) {

import DPipeOps._

val r8 = 100.0 !> {x : Double => x / 0.0}

println(r8)

val r9 = r8 #> {x:Double => x* 3.0}

println(r9)

/*

val r8 = 100.0 !> { x: Double => x / 0.0 }

println( r8.get )

val r9 = r8 &> { x: Double => x * 3.0 }

println( r9 )*/

val r10 = (100.0 !> {x : Double => x / 0.0}) #> {x:Double => x* 3.0}

//val r10 = ( 100.0 !> { x: Double => x / 0.0 } ) &> { x: Double => x * 3.0 }

val r11 = 100.0 !> {x : Double => x / 0.0} #> {x:Double => x* 3.0}

//val r11 = 100.0 !> { x: Double => x / 0.0 } &> { x: Double => x * 3.0 }

}

}

As it stands we have the following error in the last code line:

`Cannot prove that Double => Double =:= scala.util.Try[Double].`

val r11 = 100.0 !> {x : Double => x / 0.0} #> {x:Double => x* 3.0}

^

Notice that in the second last code line, I need only add

the parenthesis to enforce left left-hand associativity

(Scala default). It seems like the

`#>`

use the function

`{x : Double => x / 0.0}`

Double.

If however I use the "&>" operator, no error occurs. In the

test code below, just flip the comments. So my question is, why

is this happening. Is this something new to Scala 2.12.0?

TIA

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Answer Source

The problem is with the operator precedence. Here are more details: Operator precedence in Scala

It depends on the first character of the operator. `#`

has higher precedence than `!`

and `!`

has higher precedence than `&`

.

Hence you get

```
100.0 !> ({x : Double => x / 0.0} #> {x:Double => x* 3.0})
```

instead of

```
(100.0 !> {x : Double => x / 0.0}) #> {x:Double => x* 3.0}
```

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