Assert Protocol Payload

Quality assessment is not only about status of I/O protocols but also assert of the response content. The SDK defines generic category. Its primitives acts as lense they are focuses inside the structure, fetches values and asserts them. These helpers abort the evaluation of “program” if expectations do not match actual response.

Let’s look on these primitives:

  1. Assert Protocol Payload
    1. Imports
    2. Composition
    3. Focus on the value
      1. Focus on the sequence
      2. Custom quality check


Import asserts from the cats module:

import (
  ç ""


Each assert is a “generic” arrow of type assay.Arrow. Use its compose functions

  http.Join(/* ... */),

http.Join(/* ... */).Then(

Focus on the value

var data MyType
  /* ... */
  // checks if the value is defined (shall use pointer)

  // checks if the value matches expected one
  ç.Value(&data).Is(&MyType{Site: "site", Host: "host"}),
  // checks if the value matches string literal
  ç.Value(/* ... */).Bytes([]byte{1, 2, 3, 4}),

Focus on the sequence

ç.Value lens has a limitation while working with sequences. It is not able to focus into distinct element, you should use ç.Seq instead.

var data Seq

// lookups element in the sequence by key
// lookups element in the sequence and matches it against expected value
ç.Seq(&data).Has("site", MyType{Site: "site", Host: "host"})

Your data type needs to implement assay.Ord interface.

type Ord interface {
  // String return primary key as string type
  String(int) string
  // Value return value at index
  Value(int) interface{}

assay.Ord extends sort.Interface with ability to lookup element by string. In this example, the interface returns sequence of elements. The lens Seq and Has focuses on the required element. A reference implementation of the interface is

  type Seq []MyType

  func (seq Seq) Len() int                { return len(seq) }
  func (seq Seq) Swap(i, j int)           { seq[i], seq[j] = seq[j], seq[i] }
  func (seq Seq) Less(i, j int) bool      { return seq[i].Site < seq[j].Site }
  func (seq Seq) String(i int) string     { return seq[i].Site }
  func (seq Seq) Value(i int) interface{} { return seq[i] }

Custom quality check

Often, you need to write a small function to apply a custom check on the received value. These custom functions shall be composable within the category pattern. Let’s consider a previous example.

Your assessment has successfully received the sequence of elements into seq variable. We need to write a small function that extracts first elements from the sequence. A following traditional coding style does not work

var seq Seq
  /* ... */
head := seq[0]

The sequence is not “materialized” yet at the moment when http.Join(...) returns. It only returns a “promise” of HTTP I/O which is materialized later. Therefore, any computation have to be lifted-and-composed with this promise. ç.FMap does it. ç.FMap takes a closure and applies it to the results of network communication:

var seq Seq
var head MyType
  /* ... */
  ç.FMap(func() error {
    head := seq[0]
    return nil

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