Encoding floats with transit-cljs

Question

ClojureScript will represent numeric values like {:person/weight 200.0} using Javascript's only numeric type, Number. When such a record is serialized with transit, transit sees Number: 200 and packs that as an integer, and then other platforms e.g. JVM will unpack that as an integer, not a float, and now systems like Datomic will crash on the unexpected type unless I preprocess the values according to a schema that says :person/weight is in fact :db.type/float.

Is this a design flaw in transit? Should transit be changed to allow me to specify the type and/or integrate with a schema provider (spec) to choose the right type?

Answer 1

Just trying to restate your question a bit, I believe you are asking, is there some way for transit-cljs (or really transit-js) to forcibly encode the Javascript number 200.0 as transit ground type "floating point decimal"? (This isn't really a transit-format question, as transit itself does have the capability to represent both fixed and arbitrary precision integers and floats. So, this is really a language specific encoder question.)

I think the answer is no (but I am not an expert in the js side of this world). transit-js/transit-cljs only encode as transit ground types i (integer, signed 64 bit) or f (arbitrary precision decimal). I don't know, but I would guess this is because it's difficult to tell whether a number is representable as a floating point decimal.

Presumably it's possible to allow you to force such a thing, but I think the best place to have that discussion is probably in either https://github.com/cognitect/transit-js/issues or https://github.com/cognitect/transit-cljs/issues.

Comments

I'm questioning at the philosophical level that Transit cannot possibly do its job correctly [cross platform value interchange] unless it is integrated with a schema provider like spec or Datomic?

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The whole point of transit is to be a schemaless means of transmitting values by providing a means to encode (and extend types). Per the rationale, "Transit processes elements in terms of semantic types, but it is not a type system, and has no schemas. " https://github.com/cognitect/transit-format#rationale Can transit transmit floating point values? Yes, it has a ground type for that.

Questions about per-language encoding or decoding of transit types is a question for those language-specific encoders or decoders (as the language-specific capabilities vary). Does transit-cljs/transit-js have a means to encode Javascript numbers as floating point decimals? No, not currently.

These questions have nothing to do with either schemas or Datomic afaict.

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After thought, I think Transit's stated goal of being schemaless is about decoding and interpreting. You don't need a schema to read it. However in this particular case, the schema is essential complexity in helping a quirky platform on the write side. IMO

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In this case, you've lost the information when deserializing to JS, because its type system can't model the distinction. So that number is effectively an integer now.

(Number/isInteger 200.0)
;> true

So from the serialization perspective, it seems correct.

What seems to he happening is your database has a strict schema. So I'd simply implement coercing at that level.