decimal: add decomposer interface

[kardianos]

For golang/go#30870

[kardianos]

Please let me know what you think of this idea. See referenced issue.

[njason]

Can these test cases include expected results which are then tested against the actual results with assert.Equal? It would be good to have more test cases as well

[kardianos]

I don't see any uses of an "assert" package, but, I added more test cases, it already checks with Decimal.Cmp and I added a check to the original S. Because this is a round trip test, I'm checking the final decimal against the original parsed number.

[njason]

These forms should constants rather than magic numbers

[kardianos]

I added consts above. The interface is designed to be a byte type to allow it to not rely on an external package for types.

[njason]

Why even include these cases, or have the form argument at all, if they cannot be accepted?

[kardianos]

This interface is designed to convert between many different decimal packages. Some of these other packages handle NaN and and Infinite forms. This is to have explicit error messages for messaging.

[kardianos]

I removed the use of testing.T.Run, which was introduced in Go1.7. CI passes now.

[kardianos]

Ping.

[kardianos]

Ping

[kardianos]

@njason Can we resume this conversation? This is in Go1.13 and crdb/apd and ericlagergren/decimal.

[njason]

@kardianos I see this has been added as an experimental interface to 1.13 which is in an unstable release state. Please circle back when this is in a stable release.

[kardianos]

1.13 has now been released.

[njason]

Can we remove the decomposer interface in this PR since it's already defined in core?

[kardianos]

The most recent build on TravisCI complained about Go1.2 being missing. Looking on https://golang.org/dl/ Go 1.2.2 is present, but Go1.2 is indeed gone. To test, I changed the travis ci file and it passed using Go1.2.2.

[kardianos]

Also, I removed the interface per your request.

[5teven]

Hello everyone

After this update i got error:
pq: encode: unknown type for decimal.Decimal

I'm using:
go1.13 linux/amd64
postgresql 10
sqlx

[kardianos]

@5teven What is the postgresql driver version? Also, please open a new issue at golang.org/issue and @ me.

[tietang]

After this update i got error:
cannot convert type: decimal.Decimal

go 1.13.4
go-sql-driver/mysql 1.4.1

i think go mysql driver can't support decimalDecompose.

please see:

1. database/sql/driver.IsValue, line 176:
   // IsValue reports whether v is a valid Value parameter type. func IsValue(v interface{}) bool { if v == nil { return true } switch v.(type) { case []byte, bool, float64, int64, string, time.Time: return true case decimalDecompose: return true } return false }

2.go-sql-driver/mysql statement.go line 140, method ConvertValue:
line 141:
`
if driver.IsValue(v) {
return v, nil
}

if vr, ok := v.(driver.Valuer); ok {
	sv, err := callValuerValue(vr)
	if err != nil {
		return nil, err
	}
	if !driver.IsValue(sv) {
		return nil, fmt.Errorf("non-Value type %T returned from Value", sv)
	}
	return sv, nil
}

`

3. go-sql-driver/mysql packets.go line 1071

line 1071
default: return fmt.Errorf("cannot convert type: %T", arg) }

[njason]

commit 75bb2cb has been reverted from master and will be worked on in another branch