So interfaces are used because the typescript docs recommend using them over type aliases whenever possible (primarily because they're extensible).

Interfaces were chosen before graphql union/interfaces were actually implemented. The type aliases were used for interface/unions, because I found having the union type for all implementations extremely useful.

Is there a downside to using interfaces that I'm unaware of? Perhaps we should look into alternatives for flow if there are

Its not so much the interface, as the conflicting name and the lack of options (at least from what I found) around the union type. For my situation what would be nice:

• graphql types and interfaces exported to flow interface with original name
• graphql implementations exported to flow union with name like: interface + Implementation

Hey, so I took a stab at building a more "flow-esque" type generation format that matches the nomenclature that flow tends to use. This is the gist of it (using lodash, and node require instead of es modules). If there is interest, I'd be happy to open a PR with these changes to the packages/flow-language settings:

const { camelCase, upperFirst } = require('lodash')

const pascalize = str => upperFirst(camelCase(str))

const { DEFAULT_OPTIONS: TS_OPTIONS } = require('@gql2ts/language-typescript')

const FLOW_WRAP_PARTIAL = partial => `$SHAPE<${partial}>`
const FLOW_INTERFACE_NAMER = name => `${pascalize(name)}`
const FLOW_INTERFACE_BUILDER = (name, body) => `export type ${name} = ${body}`
const FLOW_ENUM_NAME_GENERATOR = name => `${pascalize(name)}`
const FLOW_TYPE_PRINTER = (type, isNonNull) => (isNonNull ? type : `?${type}`)

const FLOW_POST_PROCESSOR = str => `/* @flow */
${str}
`
const FLOW_NAMESPACE_GENERATOR = (namespaceName, interfaces) => `
// graphql flow definitions
${interfaces}
`

const DEFAULT_OPTIONS = {
  ...TS_OPTIONS,
  printType: FLOW_TYPE_PRINTER,
  generateInterfaceName: FLOW_INTERFACE_NAMER,
  generateEnumName: FLOW_ENUM_NAME_GENERATOR,
  interfaceBuilder: FLOW_INTERFACE_BUILDER,
  generateNamespace: FLOW_NAMESPACE_GENERATOR,
  wrapPartial: FLOW_WRAP_PARTIAL,
  postProcessor: FLOW_POST_PROCESSOR,
}

module.exports = DEFAULT_OPTIONS

@arahansen looks good to me, a PR would definitely be welcome 😄

FWIW you can also use pascalize from humps as it's already depended on by @gql2ts/language-typescript