Decode: allow integers to be unmarshaled into floats

[PotatoesFall]

This issue was raised and is described in https://github.com/pelletier/go-toml/issues/835

Float values do not require a floating point according to toml documentation: https://toml.io/en/v1.0.0#float

This PR allows integer toml values to be decoded into float64 and float32 fields.

There is no conversion check but as far as I can tell any int64 value can always be converted to a float32 or float64, albeit with potential loss of precision.

I am not sure I got the style right when it comes to tests - I was looking for test(s) where all possible types of values for a field are unmarshaled but found no such test.

---

stdout of ./ci.sh coverage -d v2 :

/tmp/tmp.MGcFIXUKIi ~/github.com/pelletier/go-toml
total:  (statements)    0.0%
~/github.com/pelletier/go-toml
/tmp/tmp.aycJKpzKVa ~/github.com/pelletier/go-toml
ok      github.com/pelletier/go-toml/v2 0.026s  coverage: 86.3% of statements in ./...
ok      github.com/pelletier/go-toml/v2/benchmark   0.467s  coverage: 41.5% of statements in ./...
?       github.com/pelletier/go-toml/v2/cmd/gotoml-test-decoder [no test files]
ok      github.com/pelletier/go-toml/v2/cmd/jsontoml    0.007s  coverage: 5.1% of statements in ./...
ok      github.com/pelletier/go-toml/v2/cmd/tomljson    0.007s  coverage: 17.6% of statements in ./...
ok      github.com/pelletier/go-toml/v2/cmd/tomll   0.005s  coverage: 19.2% of statements in ./...
?       github.com/pelletier/go-toml/v2/cmd/tomltestgen [no test files]
?       github.com/pelletier/go-toml/v2/internal/characters [no test files]
ok      github.com/pelletier/go-toml/v2/internal/cli    0.005s  coverage: 13.7% of statements in ./...
ok      github.com/pelletier/go-toml/v2/internal/danger 0.005s  coverage: 1.0% of statements in ./...
ok      github.com/pelletier/go-toml/v2/internal/imported_tests 0.007s  coverage: 55.2% of statements in ./...
?       github.com/pelletier/go-toml/v2/internal/testsuite  [no test files]
ok      github.com/pelletier/go-toml/v2/internal/tracker    0.005s  coverage: 0.1% of statements in ./...
ok      github.com/pelletier/go-toml/v2/unstable    0.004s  coverage: 12.2% of statements in ./...
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github.com/pelletier/go-toml/v2/decode.go:12:               parseInteger                    85.7%
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github.com/pelletier/go-toml/v2/decode.go:78:               parseDateTime                   97.4%
github.com/pelletier/go-toml/v2/decode.go:160:              parseLocalDateTime              100.0%
github.com/pelletier/go-toml/v2/decode.go:191:              parseLocalTime                  97.9%
github.com/pelletier/go-toml/v2/decode.go:283:              parseFloat                  100.0%
github.com/pelletier/go-toml/v2/decode.go:333:              parseIntHex                 100.0%
github.com/pelletier/go-toml/v2/decode.go:347:              parseIntOct                 100.0%
github.com/pelletier/go-toml/v2/decode.go:361:              parseIntBin                 100.0%
github.com/pelletier/go-toml/v2/decode.go:375:              isSign                      100.0%
github.com/pelletier/go-toml/v2/decode.go:379:              parseIntDec                 100.0%
github.com/pelletier/go-toml/v2/decode.go:403:              checkAndRemoveUnderscoresIntegers       100.0%
github.com/pelletier/go-toml/v2/decode.go:452:              checkAndRemoveUnderscoresFloats         100.0%
github.com/pelletier/go-toml/v2/decode.go:514:              isValidDate                 100.0%
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github.com/pelletier/go-toml/v2/internal/danger/typeid.go:18:       MakeTypeID                  100.0%
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github.com/pelletier/go-toml/v2/localtime.go:57:            String                      100.0%
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github.com/pelletier/go-toml/v2/localtime.go:101:           String                      100.0%
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github.com/pelletier/go-toml/v2/localtime.go:111:           UnmarshalText                   100.0%
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github.com/pelletier/go-toml/v2/marshaler.go:229:           isRoot                      100.0%
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github.com/pelletier/go-toml/v2/marshaler.go:522:           encodeTableHeader               100.0%
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github.com/pelletier/go-toml/v2/marshaler.go:612:           sortEntriesByKey                100.0%
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github.com/pelletier/go-toml/v2/marshaler.go:702:           encodeStruct                    100.0%
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github.com/pelletier/go-toml/v2/marshaler.go:729:           isValidName                 83.3%
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github.com/pelletier/go-toml/v2/marshaler.go:783:           encodeTable                 97.2%
github.com/pelletier/go-toml/v2/marshaler.go:848:           encodeTableInline               88.2%
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github.com/pelletier/go-toml/v2/unmarshaler.go:52:          DisallowUnknownFields               100.0%
github.com/pelletier/go-toml/v2/unmarshaler.go:98:          Decode                      100.0%
github.com/pelletier/go-toml/v2/unmarshaler.go:151:         typeMismatchError               80.0%
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github.com/pelletier/go-toml/v2/unmarshaler.go:164:         nextExpr                    100.0%
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github.com/pelletier/go-toml/v2/unmarshaler.go:176:         arrayIndex                  100.0%
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github.com/pelletier/go-toml/v2/unstable/parser.go:565:         parseMultilineLiteralString         100.0%
github.com/pelletier/go-toml/v2/unstable/parser.go:584:         parseMultilineBasicString           98.5%
github.com/pelletier/go-toml/v2/unstable/parser.go:715:         parseKey                    100.0%
github.com/pelletier/go-toml/v2/unstable/parser.go:758:         parseSimpleKey                  100.0%
github.com/pelletier/go-toml/v2/unstable/parser.go:780:         parseBasicString                100.0%
github.com/pelletier/go-toml/v2/unstable/parser.go:875:         hexToRune                   100.0%
github.com/pelletier/go-toml/v2/unstable/parser.go:904:         parseWhitespace                 100.0%
github.com/pelletier/go-toml/v2/unstable/parser.go:914:         parseIntOrFloatOrDateTime           100.0%
github.com/pelletier/go-toml/v2/unstable/parser.go:962:         scanDateTime                    100.0%
github.com/pelletier/go-toml/v2/unstable/parser.go:1028:        scanIntOrFloat                  100.0%
github.com/pelletier/go-toml/v2/unstable/parser.go:1116:        isDigit                     100.0%
github.com/pelletier/go-toml/v2/unstable/parser.go:1122:        isValidHexRune                  100.0%
github.com/pelletier/go-toml/v2/unstable/parser.go:1129:        isValidOctalRune                100.0%
github.com/pelletier/go-toml/v2/unstable/parser.go:1133:        isValidBinaryRune               100.0%
github.com/pelletier/go-toml/v2/unstable/parser.go:1137:        expect                      100.0%
github.com/pelletier/go-toml/v2/unstable/scanner.go:5:          scanFollows                 100.0%
github.com/pelletier/go-toml/v2/unstable/scanner.go:11:         scanFollowsMultilineBasicStringDelimiter    100.0%
github.com/pelletier/go-toml/v2/unstable/scanner.go:15:         scanFollowsMultilineLiteralStringDelimiter  100.0%
github.com/pelletier/go-toml/v2/unstable/scanner.go:19:         scanFollowsTrue                 100.0%
github.com/pelletier/go-toml/v2/unstable/scanner.go:23:         scanFollowsFalse                100.0%
github.com/pelletier/go-toml/v2/unstable/scanner.go:27:         scanFollowsInf                  100.0%
github.com/pelletier/go-toml/v2/unstable/scanner.go:31:         scanFollowsNan                  100.0%
github.com/pelletier/go-toml/v2/unstable/scanner.go:35:         scanUnquotedKey                 100.0%
github.com/pelletier/go-toml/v2/unstable/scanner.go:46:         isUnquotedKeyChar               100.0%
github.com/pelletier/go-toml/v2/unstable/scanner.go:50:         scanLiteralString               100.0%
github.com/pelletier/go-toml/v2/unstable/scanner.go:71:         scanMultilineLiteralString          100.0%
github.com/pelletier/go-toml/v2/unstable/scanner.go:128:        scanWindowsNewline              100.0%
github.com/pelletier/go-toml/v2/unstable/scanner.go:141:        scanWhitespace                  100.0%
github.com/pelletier/go-toml/v2/unstable/scanner.go:155:        scanComment                 100.0%
github.com/pelletier/go-toml/v2/unstable/scanner.go:183:        scanBasicString                 100.0%
github.com/pelletier/go-toml/v2/unstable/scanner.go:210:        scanMultilineBasicString            100.0%
total:                                  (statements)                    97.0%
~/github.com/pelletier/go-toml

Results: v2 0.0% HEAD 97.0%
Delta: 97.0

[pelletier]

Thank you for the pull request! I was actually looking into this the other day. It's something that go-toml v1 used to allow, but I removed in go-toml v2. My reasoning was to avoid the loss of precision, and it's kind of a slippery slope to allow automatic conversions.

Can you point out in the specification where floats can be expressed like integers? I'm seeing this:

A float consists of an integer part (which follows the same rules as decimal integer values) followed by a fractional part and/or an exponent part.

Anyway, I think I'm coming around to it, given that encoding/json also permits it.

[PotatoesFall]

For me it's this one:

The decimal point, if used, must be surrounded by at least one digit on each side.

Specifically, the "if used" seems to imply that this is allowed.

On closer inspection, I do now see the "and/or" wording for decimal points and exponents, so this could refer to the case where we have an exponent, so I think I misread the docs, oops.

I don't think the comparison with encoding/json should have an influence here, since integers and floats are all the same number data type in JSON, unlike TOML.

That being said, I think it does make sense to allow this. There is no exact mapping of data types from TOML to go, so when unmarshalling I think the question should really be if the value can be assigned in a sensible way. In the case of integers, it generally is assignable to a float right?

The argument that we lose precision also seems inconsistent, since we also lose precision with floats:

• most decimals cannot be expressed in binary
• strconv.ParseFloat allows values with much higher precision than a float64 can hold, with 100 decimal digits for example.

[pelletier]

the question should really be if the value can be assigned in a sensible way. In the case of integers, it generally is assignable to a float right?

The thing that worries me is the precision loss for larger integers into floats (16777217 for float32 and 9007199254740993 for float64), since they start being less precise.

On the one hand, we could implement a bound check for those values and error out, or we could still do it and put it on the user to know what they are doing. One of the guidelines for this project is "no surprises", but I think both behaviors could be surprising. What do you think?

[PotatoesFall]

One of the guidelines for this project is "no surprises", but I think both behaviors could be surprising. What do you think?

I definitely agree with "no surprises" ! This does need some discussion. I'll invent some cases for illustration.

Surprise from current state

• I take number values from JSON, expecting them to fit into a float. some of them do, some of them don't.
• I have a value that I run through strconv.ParseFloat and it works fine. I put it in my toml and it says it's not a float.
• I know that the value 1 fits into a float - I change a setting in a TOML config file from 1.5 to 1 and suddenly my service crashes.

Surprise from accepting all ints even with precision loss

• I parse a TOML file and then export it. One of the values seems to have randomly changed.

Surprise from accepting ints but only without precision loss

• I check to see if I can unmarshal an int into a float, it works and I build my program around this. One day the output of my toml config generator becomes too large and my program unexpectedly crashes.
• I have a value that I run through strconv.ParseFloat and it works fine. I put it in my toml and it says it doesn't fit into a float64.
• I can put all these values into a float:
  • :heavy_check_mark: 12341234123412341234.12341234123412341234
  • :heavy_check_mark: 12341234123412341234.0
  • :x: 12341234123412341234

My personal and totally biased conclusion is that allowing ints and accepting precision loss has much less confusing surprises, and stays consistent with the behavior of the standard library. The latter is also the reason I think this is the best toml package out there.

EDIT upon writing this I realized that parsing values like 12341234123412341234 would fail since they don't fit into an int64, I pushed a fix for that.

[pelletier]

I think you're right, this seems like the right tradeoff for this lib. Thank you for the patch and the discussion!