# Five Go Interop — Using Go Packages Directly from PRG

Five's key differentiator: **use Go's entire package ecosystem directly from PRG code**.

## 1. IMPORT — Bring Go Packages into PRG

```prg
IMPORT "strings"                    // Go standard library
IMPORT "database/sql"               // Database
IMPORT "net/http"                   // HTTP server/client
IMPORT "encoding/json"              // JSON
IMPORT _ "modernc.org/sqlite"       // blank import (driver registration)
IMPORT myhttp "net/http"            // aliased import
```

Declare at the top of PRG file. gengo converts directly to Go imports.

## 2. Package Function Calls — `pkg.Func()`

```prg
IMPORT "strings"
IMPORT "strconv"
IMPORT "fmt"

PROCEDURE Main()
   LOCAL cResult, nVal, cFormatted

   cResult := strings.ToUpper("hello five!")     // "HELLO FIVE!"
   cResult := strings.ReplaceAll("a-b-c", "-", "_")  // "a_b_c"

   nVal := strconv.Atoi("42")                    // 42
   cFormatted := fmt.Sprintf("Name: %s, Age: %d", "Charles", 30)

   IF strings.HasPrefix(cResult, "HELLO")
      ? "starts with HELLO"
   ENDIF

   RETURN
```

### How It Works
```
PRG:  strings.ToUpper("hello")
  ↓ gengo
Go:   hbrt.GoCallFast(_ff_strings_ToUpper, _arg0)
```

- gengo detects imported package names
- `pkg.Func(args)` → `hbrt.GoCallFast()` type-specialized call
- Return values automatically converted to Harbour Values

### Automatic Type Conversion

| Go Type | → Harbour Type |
|---------|---------------|
| `string` | String |
| `int`, `int64` | Numeric (Integer/Long) |
| `float64` | Numeric (Double) |
| `bool` | Logical |
| `[]string`, `[]int` etc. | Array |
| `map[string]interface{}` | Hash |
| `error` (nil) | NIL |
| `error` (non-nil) | String (error message) |
| `*sql.DB` etc. (pointer) | Go Object (wrapped in Value) |

## 3. Go Object Method Calls — `obj:Method()`

Go function return objects are called with Harbour's `:` syntax:

```prg
IMPORT "database/sql"
IMPORT _ "modernc.org/sqlite"

PROCEDURE Main()
   LOCAL db, rows

   db := sql.Open("sqlite", ":memory:")
   db:Exec("CREATE TABLE test (id INTEGER)")
   db:Exec("INSERT INTO test VALUES (1)")

   rows := db:Query("SELECT * FROM test")
   DO WHILE rows:Next()
      ? rows:Column(1)
   ENDDO
   rows:Close()
   db:Close()

   RETURN
```

### How It Works
```
PRG:  db:Exec("CREATE TABLE ...")
  ↓ gengo
Go:   if hbrt.IsGoObject(_obj) {
          hbrt.GoCallCached(_obj, "Exec", _args...)  // reflect + cache
      } else {
          t.Send("Exec", 1)                           // Harbour object
      }
```

- Runtime auto-detection: Go object vs Harbour object
- Go object: `reflect.MethodByName()` with method cache
- Harbour object: existing `Send()` mechanism

## 4. Multiple Go Objects Simultaneously

```prg
IMPORT "database/sql"
IMPORT _ "modernc.org/sqlite"

PROCEDURE Main()
   LOCAL dbSource, dbTarget, aRows, i

   dbSource := sql.Open("sqlite", "source.db")
   dbTarget := sql.Open("sqlite", "target.db")

   aRows := SqlScan(dbSource, "SELECT * FROM products")
   FOR i := 1 TO Len(aRows)
      dbTarget:Exec("INSERT INTO inventory VALUES (...)")
   NEXT

   dbSource:Close()
   dbTarget:Close()
   RETURN

FUNCTION SqlScan(db, cSQL)
   LOCAL rows, cols, aResult, aRow, i, nCols
   aResult := {}
   rows := db:Query(cSQL)
   cols := rows:Columns()
   nCols := Len(cols)
   DO WHILE rows:Next()
      aRow := {=>}
      FOR i := 1 TO nCols
         aRow[cols[i]] := rows:Column(i)
      NEXT
      AAdd(aResult, aRow)
   ENDDO
   rows:Close()
   RETURN aResult
```

## 5. Array Return Handling

Go functions returning slices automatically convert to Harbour arrays:

```prg
IMPORT "strings"

PROCEDURE Main()
   LOCAL aParts, i

   aParts := strings.Split("one,two,three", ",")

   ? Len(aParts)        // 3
   ? aParts[1]          // "one"
   ? aParts[2]          // "two"
   ? aParts[3]          // "three"

   FOR i := 1 TO Len(aParts)
      ? "  [" + Str(i, 1) + "]", aParts[i]
   NEXT
   RETURN
```

## 6. #pragma BEGINDUMP — Advanced Use (Optional)

Only needed for complex Go logic:

```prg
PROCEDURE Main()
   ? MyGoFunc("hello")
   RETURN

#pragma BEGINDUMP
import "five/hbrt"

func init() {
   hbrt.HB_FUNC("MYGOFUNC", func(ctx *hbrt.HBContext) {
      s := ctx.ParC(1)
      ctx.RetC(strings.ToUpper(s) + "!!!")
   })
}
#pragma ENDDUMP
```

### HB_FUNC API (Harbour C API Compatible)

| Harbour C | Five Go | Description |
|-----------|---------|-------------|
| `HB_FUNC(NAME)` | `hbrt.HB_FUNC("NAME", fn)` | Register function |
| `hb_pcount()` | `ctx.PCount()` | Parameter count |
| `hb_parc(n)` | `ctx.ParC(n)` | String parameter |
| `hb_parni(n)` | `ctx.ParNI(n)` | Integer parameter |
| `hb_parnl(n)` | `ctx.ParNL(n)` | Long parameter |
| `hb_parnd(n)` | `ctx.ParND(n)` | Double parameter |
| `hb_parl(n)` | `ctx.ParL(n)` | Logical parameter |
| `hb_pards(n)` | `ctx.ParDS(n)` | Date (YYYYMMDD) |
| `HB_ISCHAR(n)` | `ctx.IsChar(n)` | Type check |
| `HB_ISNUM(n)` | `ctx.IsNum(n)` | Type check |
| `hb_retc(s)` | `ctx.RetC(s)` | Return string |
| `hb_retni(n)` | `ctx.RetNI(n)` | Return integer |
| `hb_retnd(d)` | `ctx.RetND(d)` | Return double |
| `hb_retl(b)` | `ctx.RetL(b)` | Return logical |
| `hb_storc(s,n)` | `ctx.StorC(s,n)` | By-ref store |
| `hb_arrayNew()` | `ctx.ArrayNew(n)` | Create array |
| `hb_arrayGet()` | `ctx.ArrayGet(v,i)` | Array read |
| `hb_hashNew()` | `ctx.HashNew()` | Create hash |

### Five Extension API (Go-specific, not in Harbour)

| API | Description |
|-----|-------------|
| `ctx.ParDate(n)` | Returns `time.Time` |
| `ctx.ParArray(n)` | Returns `[]Value` |
| `ctx.ParHash(n)` | Returns `*HbHash` |
| `ctx.RetArray(items)` | Return array |
| `ctx.RetHash(h)` | Return hash |
| `ctx.RetVal(v)` | Return any Value |
| `hbrt.WrapGo(obj)` | Go object → Value |
| `hbrt.UnwrapGo(v)` | Value → Go object |
| `hbrt.GoCall(v, method, args...)` | Reflect method call |

## 7. Available Go Packages

| Package | PRG Usage Example |
|---------|-------------------|
| `strings` | `strings.ToUpper()`, `strings.Split()`, `strings.Contains()` |
| `strconv` | `strconv.Atoi()`, `strconv.FormatFloat()` |
| `fmt` | `fmt.Sprintf()` |
| `database/sql` | `sql.Open()` → `db:Exec()`, `db:Query()` |
| `net/http` | HTTP server, REST API |
| `encoding/json` | JSON encode/decode |
| `os` | `os.ReadFile()`, `os.Stat()` |
| `path/filepath` | `filepath.Join()`, `filepath.Glob()` |
| `time` | `time.Now()`, `time.Since()` |
| `crypto/sha256` | Hash functions |
| `regexp` | Regular expressions |
| `sort` | Sorting |
| External | `modernc.org/sqlite`, `github.com/...` etc. |

## 8. Core Principles

1. **IMPORT is all you need** — No #pragma BEGINDUMP required
2. **100% PRG code** — Zero Go code to use Go features
3. **Automatic type conversion** — string/int/bool/array/hash bidirectional
4. **Transparent Go objects** — Store in LOCAL, call with `:`
5. **Harbour compatible** — Existing xBase syntax unchanged, Go is the backend