E4101

E4101#

Unsupported expression after the pipe operator.

This error occurs when the expression after the pipe operator (|>) is not in a supported form. The pipe operator allows you to chain function calls in a more readable way, but only supports specific forms on its right-hand side.

The following forms are allowed after the pipe operator:

  1. A single identifier (function name)

  2. A regular function application (but not method calls)

  3. A constructor name

  4. A constructor application

Erroneous example#

///|
pub struct T(Int)

///|
pub fn make(x : Int) -> T {
  T(x)
}

///|
fn T::m(self : T, x : Int) -> Unit {
  println(self.0 + x)
}

///|
pub fn f(t : T, x : Int) -> Unit {
  x |> t.m()
  //     ^^^^ Error: Unsupported expression after the pipe operator.
}

Suggestion#

To fix this error, you can change the invalid pipe expression to normal function or method application.

///|
pub struct T(Int)

///|
pub fn make(x : Int) -> T {
  T(x)
}

///|
fn T::m(self : T, x : Int) -> Unit {
  println(self.0 + x)
}

///|
pub fn f(t : T, x : Int) -> Unit {
  (fn(x : Int) { println(x) })(x)
  t.m(x)
}
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