[Request]: Require Type Definitions First

[bradennapier]

I can see it being a good rule to enforce all type definitions being at the top of the file.

good

/* @flow */

type MyType = number

const b: MyType = 2

function foo(): MyType {
     return b
}

bad

/* @flow */

function foo(): MyType {
     return b
}

type MyType = number 

const b: MyType = 2

[pnevyk]

Related to #168