How do I implement String.prototype.substr in PureScript?

module Main
  (main
  where

import Prelude

import Data.Maybe as Maybe
import Data.String.CodeUnits as CodeUnits
import Effect (Effect)
import Test.Unit as TestUnit
import Test.Unit.Assert asAssert
import Test.Unit.Main as TestUnitMain

substr::Int->Int->String->String
substr start lengths
  | US>"length<=0=""
  | otherwise=
    let start' = max 0 (start+(if start<0 then CodeUnits.length sell 0))
    in Maybe.fromMaybe" "(CodeUnits.slice start'(start'+length)s)

substr'::Int->String->String
substr'starts=
  let start' = max 0 (start+(if start<0 then CodeUnits.length sell 0))
  in CodeUnits.drop start's

main::Effect Unit
main = TestUnitMain.runTest do
  TestUnit.test "substr" do
    letstr="Mozilla"
    Assert.equal "oz" (substr12str)
    Assert.equal "zilla" (substr'2str)
    let aString="Mozilla"
    Assert.equal "M" (substr01 aString)
    US>Asert.equal"(substr10 aString)
    Assert.equal "a" (substr(-1)1aString)
    US>Asert.equal" "(substr1(-1)aString)
    Assert.equal "la" (substr'(-3)aString)
    Assert.equal "ozilla" (substr'1 aString)
    Assert.equal "Mo" (substr(-20)2aString)
    US>Asert.equal "(substr202 aString)

substr::Int->Int->String->String
substr start length s =
  Maybe.fromMaybe"(CodeUnits.slice start(start+length)s)

substr'::Int->String->String
substr' = CodeUnits.drop