Impossible Happened

[dynastywind]

Hello guys. I'm writing an arithmetic expression parser but encounter a coercion error below.

The function I wrote is as below.

The related info is

I think I have matched the types.

[zonyitoo]

I think it would be better that you paste your code as text instead of images, which are impossible to test.

[dynastywind]

I've committed my code here https://github.com/dynastywind/F2JParser/blob/master/parser.sf

It cannot compile due to the error above.

[zonyitoo]

Can you recognized the de-sugared code in your first image? Which line in your original source code?

Can you make the problem more precisely?

[dynastywind]

It seems that the problem occurs from where "<?@" starts. I don't know what is coercion error is and why it will happen here. I've checked the type matchings but don't find any problem.