fp-go-http

Development

Use this skill when making HTTP requests in fp-go using the ReaderIOResult-based HTTP client (github.com/IBM/fp-go/v2/context/readerioresult/http). Trigger on mentions of fp-go HTTP, MakeClient, MakeGetRequest, MakeRequest, ReadJSON, ReadText, ReadAll, ReadFullResponse, the HTTP request builder (WithURL, WithJSON, WithBearer, WithHeader, WithQueryArg), parallel requests with TraverseArray or TraverseTuple2, or building context-aware, composable HTTP pipelines that propagate errors through the Result monad.

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fp-go HTTP Requests

Overview

fp-go wraps net/http in the ReaderIOResult monad, giving you composable, context-aware HTTP operations with automatic error propagation. The core package is:

github.com/IBM/fp-go/v2/context/readerioresult/http

All HTTP operations are lazy — they describe what to do but do not execute until you call the resulting function with a context.Context.

Core Types

// Requester builds an *http.Request given a context.
type Requester = ReaderIOResult[*http.Request]  // func(context.Context) func() result.Result[*http.Request]

// Client executes a Requester and returns the response wrapped in ReaderIOResult.
type Client interface {
    Do(Requester) ReaderIOResult[*http.Response]
}

Basic Usage

1. Create a Client

import (
    HTTP "net/http"
    H    "github.com/IBM/fp-go/v2/context/readerioresult/http"
)

client := H.MakeClient(HTTP.DefaultClient)

// Or with a custom client:
custom := &HTTP.Client{Timeout: 10 * time.Second}
client := H.MakeClient(custom)

2. Build a Request

// GET request (most common)
req := H.MakeGetRequest("https://api.example.com/users/1")

// Arbitrary method + body
req := H.MakeRequest("POST", "https://api.example.com/users", bodyReader)

3. Execute and Parse

import (
    "context"
    H "github.com/IBM/fp-go/v2/context/readerioresult/http"
)

type User struct {
    ID   int    `json:"id"`
    Name string `json:"name"`
}

client := H.MakeClient(HTTP.DefaultClient)

// ReadJSON validates status, Content-Type, then unmarshals JSON
result := H.ReadJSON[User](client)(H.MakeGetRequest("https://api.example.com/users/1"))

// Execute — provide context once
user, err := result(context.Background())()

Response Readers

All accept a Client and return a function Requester → ReaderIOResult[A]:

Function	Returns	Notes
`ReadJSON[A](client)`	`ReaderIOResult[A]`	Validates status + Content-Type, unmarshals JSON
`ReadText(client)`	`ReaderIOResult[string]`	Validates status, reads body as UTF-8 string
`ReadAll(client)`	`ReaderIOResult[[]byte]`	Validates status, returns raw body bytes
`ReadFullResponse(client)`	`ReaderIOResult[FullResponse]`	Returns `Pair[*http.Response, []byte]`

FullResponse = Pair[*http.Response, []byte] — use pair.First / pair.Second to access components.

Composing Requests in Pipelines

import (
    F   "github.com/IBM/fp-go/v2/function"
    H   "github.com/IBM/fp-go/v2/context/readerioresult/http"
    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
    IO  "github.com/IBM/fp-go/v2/io"
)

client     := H.MakeClient(HTTP.DefaultClient)
readPost   := H.ReadJSON[Post](client)

pipeline := F.Pipe2(
    H.MakeGetRequest("https://jsonplaceholder.typicode.com/posts/1"),
    readPost,
    RIO.ChainFirstIOK(IO.Logf[Post]("Got post: %v")),
)

post, err := pipeline(context.Background())()

Parallel Requests — Homogeneous Types

Use RIO.TraverseArray when all requests return the same type:

import (
    A   "github.com/IBM/fp-go/v2/array"
    F   "github.com/IBM/fp-go/v2/function"
    H   "github.com/IBM/fp-go/v2/context/readerioresult/http"
    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
    IO  "github.com/IBM/fp-go/v2/io"
)

type PostItem struct {
    UserID uint   `json:"userId"`
    ID     uint   `json:"id"`
    Title  string `json:"title"`
}

client     := H.MakeClient(HTTP.DefaultClient)
readPost   := H.ReadJSON[PostItem](client)

// Fetch 10 posts in parallel
data := F.Pipe3(
    A.MakeBy(10, func(i int) string {
        return fmt.Sprintf("https://jsonplaceholder.typicode.com/posts/%d", i+1)
    }),
    RIO.TraverseArray(F.Flow3(
        H.MakeGetRequest,
        readPost,
        RIO.ChainFirstIOK(IO.Logf[PostItem]("Post: %v")),
    )),
    RIO.ChainFirstIOK(IO.Logf[[]PostItem]("All posts: %v")),
    RIO.Map(A.Size[PostItem]),
)

count, err := data(context.Background())()

Parallel Requests — Heterogeneous Types

Use RIO.TraverseTuple2 (or Tuple3, etc.) when requests return different types:

import (
    T   "github.com/IBM/fp-go/v2/tuple"
    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
    H   "github.com/IBM/fp-go/v2/context/readerioresult/http"
    F   "github.com/IBM/fp-go/v2/function"
)

type CatFact struct {
    Fact string `json:"fact"`
}

client         := H.MakeClient(HTTP.DefaultClient)
readPost       := H.ReadJSON[PostItem](client)
readCatFact    := H.ReadJSON[CatFact](client)

// Execute both requests in parallel with different response types
data := F.Pipe3(
    T.MakeTuple2(
        "https://jsonplaceholder.typicode.com/posts/1",
        "https://catfact.ninja/fact",
    ),
    T.Map2(H.MakeGetRequest, H.MakeGetRequest), // build both requesters
    RIO.TraverseTuple2(readPost, readCatFact),  // run in parallel, typed
    RIO.ChainFirstIOK(IO.Logf[T.Tuple2[PostItem, CatFact]]("Result: %v")),
)

both, err := data(context.Background())()
// both.F1 is PostItem, both.F2 is CatFact

Building Requests with the Builder API

For complex requests (custom headers, query params, JSON body), use the builder:

import (
    B  "github.com/IBM/fp-go/v2/http/builder"
    RB "github.com/IBM/fp-go/v2/context/readerioresult/http/builder"
    F  "github.com/IBM/fp-go/v2/function"
)

// GET with query parameters
req := F.Pipe2(
    B.Default,
    B.WithURL("https://api.example.com/items?page=1"),
    B.WithQueryArg("limit")("50"),
)
requester := RB.Requester(req)

// POST with JSON body
req := F.Pipe3(
    B.Default,
    B.WithURL("https://api.example.com/users"),
    B.WithMethod("POST"),
    B.WithJSON(map[string]string{"name": "Alice"}),
    // sets Content-Type: application/json automatically
)
requester := RB.Requester(req)

// With authentication and custom headers
req := F.Pipe3(
    B.Default,
    B.WithURL("https://api.example.com/protected"),
    B.WithBearer("my-token"),           // sets Authorization: Bearer my-token
    B.WithHeader("X-Request-ID")("123"),
)
requester := RB.Requester(req)

// Execute
result := H.ReadJSON[Response](client)(requester)
data, err := result(ctx)()

Builder Functions

Function	Effect
`B.WithURL(url)`	Set the target URL
`B.WithMethod(method)`	Set HTTP method (GET, POST, PUT, DELETE, …)
`B.WithJSON(v)`	Marshal `v` as JSON body, set `Content-Type: application/json`
`B.WithBytes(data)`	Set raw bytes body, set `Content-Length` automatically
`B.WithHeader(key)(value)`	Add a request header
`B.WithBearer(token)`	Set `Authorization: Bearer <token>`
`B.WithQueryArg(key)(value)`	Append a query parameter

Error Handling

Errors from request creation, HTTP status codes, Content-Type validation, and JSON parsing all propagate automatically through the Result monad. You only handle errors at the call site:

// Pattern 1: direct extraction
value, err := pipeline(ctx)()
if err != nil { /* handle */ }

// Pattern 2: Fold for clean HTTP handler
RIO.Fold(
    func(err error) { http.Error(w, err.Error(), http.StatusInternalServerError) },
    func(data MyType) { json.NewEncoder(w).Encode(data) },
)(pipeline)(ctx)()

Full HTTP Handler Example

package main

import (
    "context"
    "encoding/json"
    "fmt"
    "net/http"

    F   "github.com/IBM/fp-go/v2/function"
    H   "github.com/IBM/fp-go/v2/context/readerioresult/http"
    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
    IO  "github.com/IBM/fp-go/v2/io"
)

type Post struct {
    ID    int    `json:"id"`
    Title string `json:"title"`
}

var client = H.MakeClient(http.DefaultClient)

func fetchPost(id int) RIO.ReaderIOResult[Post] {
    url := fmt.Sprintf("https://jsonplaceholder.typicode.com/posts/%d", id)
    return F.Pipe2(
        H.MakeGetRequest(url),
        H.ReadJSON[Post](client),
        RIO.ChainFirstIOK(IO.Logf[Post]("fetched: %v")),
    )
}

func handler(w http.ResponseWriter, r *http.Request) {
    RIO.Fold(
        func(err error) {
            http.Error(w, err.Error(), http.StatusBadGateway)
        },
        func(post Post) {
            w.Header().Set("Content-Type", "application/json")
            json.NewEncoder(w).Encode(post)
        },
    )(fetchPost(1))(r.Context())()
}

Import Reference

import (
    HTTP "net/http"

    H   "github.com/IBM/fp-go/v2/context/readerioresult/http"
    RB  "github.com/IBM/fp-go/v2/context/readerioresult/http/builder"
    B   "github.com/IBM/fp-go/v2/http/builder"
    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
    F   "github.com/IBM/fp-go/v2/function"
    A   "github.com/IBM/fp-go/v2/array"
    T   "github.com/IBM/fp-go/v2/tuple"
    IO  "github.com/IBM/fp-go/v2/io"
)

Requires Go 1.24+.