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+# RFC: Conjure 64-bit Integer Type
+
+2019-02-14
+
+_The 54-bit Integer is Not Enough_
+
+Conjure currently supports three numeric types: `integer`, `double`, and `safelong`. Unfortunately for many use-cases, 2<sup>53</sup> - 1 is not large enough.
+
+These cases are relatively common and often result in one of three dangerous workarounds:
+1. Use safelong because it exists. We often measure nanoseconds, however a safelong can only represent three months of nanoseconds, which is long enough that it's unlikely to be caught in testing.
+1. Represent 64-bit integer values as strings in Conjure. This is only as correct as the application code which sets and parses values, which is precisely the problem Conjure is built to solve. In this case the Conjure definition fails to accurately describe the wire API.
+1. Use external type imports to polyfill the 64-bit integer type. This is incredibly dangerous, often resulting in code that behaves inconsistently between languages.
+
+## Proposal
+
+Add a new Conjure type, `integer64`, with semantics matching `integer` except for the allowed values, and JSON serialized form.
+
+For the immediate future, typescript clients may represent these values using branded strings.
+Once a serialization layer is implemented for the typescript client, they may take advantage of the `integer64`
+type using the proposed [BigInt](https://github.com/tc39/proposal-bigint) type which is already supported by
+chrome, with a [polyfill](https://github.com/GoogleChromeLabs/jsbi) for other browsers.
+
+### Allowed values
+
+The `integer64` type supports signed 64-bit values ranging from -2<sup>63</sup> to 2<sup>63</sup> - 1.
+
+### JSON Format
+
+The `integer64` type is encoded to JSON as a JSON string containing the base-10 value, otherwise javascript
+clients may transparently truncate numeric values. The [BigInt](https://github.com/tc39/proposal-bigint)
+specification does not modify how JSON is parsed, so values must use existing JSON types.
+
+For example, take a type defined as:
+
+```yml
+types:
+  definitions:
+    objects:
+      Example:
+        fields:
+          integerField: integer
+          safelongField: safelong
+          integer64Field: integer64
+```
+
+Assuming each field takes the value `123`, the above example is serialized to JSON as:
+
+```json
+{
+  "integerField": 123,
+  "safelongField": 123,
+  "integer64Field": "123"
+}
+```
+
+#### The case against using JSON numeric format
+
+We use JSON numbers elsewhere in Conjure and they, by definition, better capture the integer data type. However,
+in javascript `JSON.parse` is the most common JSON parsing function, and it results in _incorrect data_ when
+used with numeric values beyond 54-bits. Even if we do provide a serialization layer capable of handling large
+integer values, applications are still likely to attempt usage of `JSON.parse` resulting in data loss.
+
+Many existing applications already polyfill the 64-bit integer type using string encoding with the
+following block:
+
+```yml
+types:
+  imports:
+    Long:
+      base-type: string
+      external:
+        java: java.lang.Long
+```
+
+String encoded values allow these applications to upgrade and take advantage of the new type without breaking
+existing clients. Without this implementation, it would not be possible to remove support for external type
+imports without an impossibly severe API break.
+
+### PLAIN Format
+
+The PLAIN format of an `integer64` value is the base-10 numerical value.
+
+## Alternatives Considered
+
+### Arbitrary-Precision Integers
+
+While arbitrary-precision integers are more flexible, the goal of the Conjure language is to describe data as
+accurately as possible, which does not align with arbitrary precision. The need for such a type is dramatically
+less common than 64-bit values, and the existing `binary` type provides sufficient utility to represent large
+values.
+
+### Use External Type Imports
+
+Assuming java servers, and external type import for `java.lang.Long` may unblock in some cases, except it is
+difficult to guarantee correctness and compatibility with typescript clients as Conjure implementations evolve over time.
+This is not an option for non-java servers, none of which support external type imports.
+We would like to remove external type imports from Conjure entirely. Without a mechanism to support 64-bit integers
+many consumers will be unable to accept new versions of Conjure.
+
+### Serialization using JSON Numeric encoding
+
+See [The case against using JSON numeric format](#the-case-against-using-json-numeric-format).