math/big: big.Int (incorrectly) marshals to a number instead of a string

[alvarogzp]

I am using go version go1.11 linux/amd64, and reproduced this issue with the latest version (1.11).

Type Int on package "math/big" implements both [Un]marshalText and [Un]marshalJSON, the later just calling its *Text counterpart: https://golang.org/src/math/big/intmarsh.go?s=1964:2010#L64

According to json.Marshal and json.Unmarshal documentation, both functions are not equivalent. [Un]marshalJSON works with raw JSON types, but [Un]marshalText only with previously-unquoted strings. The documentation also says that [Un]marshalJSON has priority over [Un]marshalText.

So, with those premises, currently big.Int is being marshaled into a number, as their [Un]marshalJSON functions are being called and they don't quote the result after calling [Un]marshalText on the big.Int.
I think it should marshal into a string, as big.Float does (because it only implements [Un]marshalText). Also, other tools usually work and expect string representations for big numbers.

Here you can see the result of marshaling a struct with a big.Int and a big.Float: https://play.golang.org/p/WIILuWzr57S
Note how the Float is properly quoted and marshaled as a string but the Int isn't.

I propose doing one of the following things:

• Remove [Un]marshalJSON from https://golang.org/src/math/big/intmarsh.go. According to a comment, they are only there to provide "backwards compatibility" with programs that search for those methods.
• Modify big.Int's [Un]marshalJSON to quote the string it produces and unquote the one it receives, so that it mimics what json.[Un]marshal does before and after calling [Un]marshalText.
• If marshaling big.Int to a number is the desired behavior, make it explicit by removing [Un]marshalText from big.Int (as in that case, they won't apply), and also make big.Float marshal to a number too.

[slrz]

It's not possible to change the JSON marshaling behaviour of big.Ints for Go1. Even for Go2 I'm skeptical because of the breakage this would result in.

[rsc]

The JSON form of a big.Int is a json integer - not a quoted string.
The text form of a big.Int is the decimal text.
Yes, the same code generates both.
But because the JSON package prefers the JSON methods over the Text methods, there is no problem here.

Note that JSON imposes no limit on the size of decimal strings.

[alvarogzp]

@rsc what about the inconsistency with big.Float marshaler generating a quoted string?

[rsc]

If I had it to do again, I'd probably insist on big.Float having a MarshalJSON to avoid the quoting. But we missed that boat. Even for Go 2 it's not worth fixing.

[alvarogzp]

ok, thanks for the explanation!