then_keyword: if … then instead of if … do

March’s if opens its first branch with do, never then. The caret is on the then, and the fix replaces it with do.

A program that triggers it

mod Main do
  needs IO.Console
  fn sign(n : Int) : Int do
    if n > 0 then 1 else 0 end
  end

  fn main(_console : Cap(IO.Console)) do
    println(int_to_string(sign(3)))
  end
end

The fix

Replace then with do. forge fix and the editor’s quick fix apply it.

mod Main do
  needs IO.Console
  fn sign(n : Int) : Int do
    if n > 0 do 1 else 0 end
  end

  fn main(_console : Cap(IO.Console)) do
    println(int_to_string(sign(3)))
  end
end

Why the rule exists

then is the ML/OCaml spelling. March uses one block form, do … end, for every construct that takes a body, so an if branch is written the same way as a function body or a match arm block. The grammar has an explicit production for then so the message names the fix instead of reporting a generic parse failure.

See also: the language reference, elif_keyword.

See it in the error tour

• then instead of do