dotnet
Inconsistent parenthesization requirements for single-argument additional constructors (with `new`)
A single-argument additional constructor (i.e., a non-primary constructor declared with new) must have parentheses around its argument only if it is followed by another additional constructor.
Repro steps
- Define a type with multiple additional constructors, one of which takes a single atomic argument pattern.
- If the single-argument additional constructor is the last (or only) additional constructor, the argument does not need to be parenthesized; if any additional constructors follow it, the argument does need to be parenthesized.
This:
type T (x, y) =
new (x) = T (x, 3)
can be simplified to
type T (x, y) =
new x = T (x, 3)
Likewise:
type T (x, y) =
new (x, y, z) = T (x, y)
new (x) = T (x, 3)
can become
type T (x, y) =
new (x, y, z) = T (x, y)
new x = T (x, 3)
But if there's another constructor and it comes after the single-argument one:
type T (x, y) =
new (x) = T (x, 3)
new (x, y, _z) = T (x, y)
the parentheses must stay
> type T (x, y) =
- new x = T (x, 3)
- new (x, y, _z) = T (x, y);;
new (x, y, _z) = T (x, y);;
-------------------^
stdin(3,20): error FS0010: Unexpected symbol '=' in expression
The removal of the parentheses has a significant effect on the way the following additional constructors are parsed:
Expected behavior
Parentheses should either always be required for a single-argument additional constructor, or they should never be required.
Actual behavior
Parentheses are required for a single-argument additional constructor only if it is not the last additional constructor.
Known workarounds
N/A.
Related information
Provide any related information (optional):
.NET SDK 8.0.100-rc.2.23502.2 (and probably long before).
We should add a warning guiding users towards using parens https://github.com/dotnet/fsharp/issues/4704#issuecomment-380814990