A Short Guide to Pony Error Messages¶
You’ve been through the tutorial, you’ve watched some videos, and now you’re ready to write some Pony code. You fire up your editor, shovel coal into the compiler, and… you find yourself looking at a string of gibberish.
Don’t panic! Pony’s error messages try to be as helpful as possible and the ultimate goal is to improve them further. But, in the meantime, they can be a little intimidating.
This section tries to provide a short bestiary of Pony’s error messages, along with a guide to understanding them.
Let’s start with a simple one.
left side must be something that can be assigned to¶
Suppose you wrote:
actor Main let x: I64 = 0 new create(env: Env) => x = 12
The error message would be:
Error: .../a.pony:6:5: can't assign to a let or embed definition more than once x = 12 ^ Error: .../a.pony:6:7: left side must be something that can be assigned to x = 12 ^
What happened is that you declared
x as a constant, by writing
let x, and then tried to assign a new value to it, 12. To fix the error, replace
var or reconsider what value you want
x to have.
That one error resulted in two error messages. The first, pointing to the
x, describes the specific problem, that
x was defined with
let. The second, pointing to the
= describes a more general error, that whatever is on the left side of the assignment is not something that can be assigned to. You would get that same error message if you attempted to assign a value to a literal, like
cannot write to a field in a box function¶
Suppose you create a class with a mutable field and added a method to change the field:
class Wombat var color: String = "brown" fun dye(new_color: String) => color = new_color
The error message would be:
Error: .../a.pony:4:11: cannot write to a field in a box function. If you are trying to change state in a function use fun ref color = new_color ^
To understand this error message, you have to have some background. The field
color is mutable since it is declared with
var, but the method
dye does not have an explicit receiver reference capability. The default receiver reference capability is
box, which allows
dye to be called on any mutable or immutable
box reference capability says that the method may read from but not write to the receiver. As a result, it is illegal to attempt to modify the receiver in the method.
To fix the error, you would need to give the
dye method a mutable reference capability, such as
fun ref dye(new_color: String) => ....
receiver type is not a subtype of target type¶
Suppose you made a related, but slightly different error:
class Rainbow let colors: Array[String] = Array[String] fun add_stripe(color: String) => colors.push(color)
In this example, rather than trying to change the value of a field, the code calls a method which attempts to modify the object referred to by the field.
The problem is very similar to that of the last section, but the error message is significantly more complicated:
Error: ../a.pony:4:16: receiver type is not a subtype of target type colors.push(color) ^ Info: .../a.pony:4:5: receiver type: this->Array[String val] ref colors.push(color) ^ .../ponyc/packages/builtin/array.pony:252:3: target type: Array[String val] ref fun ref push(value: A): Array[A]^ => ^ .../a.pony:2:15: Array[String val] box is not a subtype of Array[String val] ref: box is not a subtype of ref let colors: Array[String] = Array[String]() ^
Once again, Pony is trying to be helpful. The first few lines describe the error, in general terms that only a programming language maven would like: an incompatibility between the receiver type and the target type. However, Pony provides more information: the lines immediately after “Info:” tell you what it believes the receiver type to be and the next few lines describe what it believes the target type to be. Finally, the last few lines describe in detail what the problem is.
Unfortunately, this message does not locate the error as clearly as the previous examples.
Breaking it down, the issue seems to be with the call to
push, with the receiver
colors. The receiver type is
this->Array[String val] ref; in other words, the view that this method has of a field whose type is
Array[String val] ref. In the class
Rainbow, the field
colors is indeed declared with the type
Array[String], and the default reference capability for
val while the default reference capability for
The “target type” in this example is the type declaration for the method
push of the class
Array, with its type variable
A replaced by
String (again, with a default reference capability of
val). The reference capability for the overall array, as required by the receiver reference capability of
ref. It seems that the receiver type and the target type should be pretty close.
But take another look at the final lines: what Pony thinks is the actual receiver type,
Array[String val] box, is significantly different from what it thinks is the actual target type,
Array[String val] ref. And a type with a reference capability of
box, which is immutable, is indeed not a subtype of a type with a reference capability of
ref, which is mutable.
The issue must lie with the one difference between the receiver type and the target type, which is the prefix “this->”. The type
this->Array[String val] ref is a viewpoint adapted type, or arrow type, that describes the
Array[String val] ref “as seen by the receiver”. The receiver, in this case, has the receiver reference capability of the method
add_stripe, which is the default
box. That is why the final type is
Array[String val] box.
The fundamental error in this example is the same as the last: the default receiver reference capability for a method is
box, which is immutable. This method, however, is attempting to modify the receiver, by adding another color stripe. That is not legal at all.
As an aside, while trying to figure out what is happening, you may have been misled by the declaration of the
let colors.... That declaration makes the
colors binding constant. As a result, you cannot assign a new array to the field. On the other hand, the array itself can be mutable or immutable. In this example, it is mutable, allowing
push to be called on the
colors field in the
A note on compiler versions¶
The error messages shown in this section are from ponyc
0.2.1-1063-g6ae110f release, the current head of the main branch at the time this is written. The messages from other versions of the compiler may be different, to a greater or lesser degree.