feat: Add samber/lo for functional collection utilities (#191)

* feat: Add samber/lo for functional collection utilities

Add samber/lo v1.52.0 which provides type-safe generic collection
operations including Map, Filter, Reduce, GroupBy, Find, and more.

This complements samber/mo (PR #189) which provides monadic types:
- samber/mo: Option, Result, Either, Future, IO
- samber/lo: Map, Filter, Reduce, GroupBy, Flatten, etc.

Together they provide a complete functional programming toolkit for Go
with compile-time type safety via generics.

* refactor: Use samber/lo for collection operations

Apply samber/lo to improve code clarity and type safety:

- payment-order/service: Use lo.Map for PaymentOrder slice transformation
- position-keeping/repository: Use lo.Filter for audit entry filtering
- platform/observability: Use lo.Keys for metadata carrier keys

These changes demonstrate the practical value of the samber/lo library
by replacing manual loops with expressive, type-safe functional operations.

* style: Fix import formatting

---------

Co-authored-by: Ben Coombs <[email protected]>

Author: bjcoombs

go.mod

@@ -42,6 +42,7 @@ require (
 	github.com/prometheus/client_model v0.6.2 // indirect
 	github.com/prometheus/common v0.66.1 // indirect
 	github.com/prometheus/procfs v0.16.1 // indirect
+	github.com/samber/lo v1.52.0 // indirect
 	github.com/spf13/pflag v1.0.10 // indirect
 	github.com/stretchr/objx v0.5.2 // indirect
 	go.yaml.in/yaml/v2 v2.4.2 // indirect

go.sum

@@ -1255,6 +1255,8 @@ github.com/rogpeppe/go-internal v1.14.1/go.mod h1:MaRKkUm5W0goXpeCfT7UZI6fk/L7L7
 github.com/russross/blackfriday/v2 v2.1.0/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQDYRxCVz55jmeOWTM=
 github.com/ruudk/golang-pdf417 v0.0.0-20181029194003-1af4ab5afa58/go.mod h1:6lfFZQK844Gfx8o5WFuvpxWRwnSoipWe/p622j1v06w=
 github.com/ruudk/golang-pdf417 v0.0.0-20201230142125-a7e3863a1245/go.mod h1:pQAZKsJ8yyVxGRWYNEm9oFB8ieLgKFnamEyDmSA0BRk=
+github.com/samber/lo v1.52.0 h1:Rvi+3BFHES3A8meP33VPAxiBZX/Aws5RxrschYGjomw=
+github.com/samber/lo v1.52.0/go.mod h1:4+MXEGsJzbKGaUEQFKBq2xtfuznW9oz/WrgyzMzRoM0=
 github.com/secure-systems-lab/go-securesystemslib v0.4.0 h1:b23VGrQhTA8cN2CbBw7/FulN9fTtqYUdS5+Oxzt+DUE=
 github.com/secure-systems-lab/go-securesystemslib v0.4.0/go.mod h1:FGBZgq2tXWICsxWQW1msNf49F0Pf2Op5Htayx335Qbs=
 github.com/serialx/hashring v0.0.0-20200727003509-22c0c7ab6b1b h1:h+3JX2VoWTFuyQEo87pStk/a99dzIO1mM9KxIyLPGTU=

internal/payment-order/service/grpc_service.go

@@ -21,6 +21,7 @@ import (
 	"github.com/meridianhub/meridian/internal/payment-order/domain"
 	poobservability "github.com/meridianhub/meridian/internal/payment-order/observability"
 	"github.com/meridianhub/meridian/internal/platform/observability"
+	"github.com/samber/lo"
 	"google.golang.org/genproto/googleapis/type/money"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/status"
@@ -1091,14 +1092,10 @@ func (s *Service) ListPaymentOrders(ctx context.Context, req *pb.ListPaymentOrde
 		return nil, status.Error(codes.Internal, "failed to list payment orders")
 	}
 
-	// Convert to proto
-	protoOrders := make([]*pb.PaymentOrder, 0, len(result.PaymentOrders))
-	for _, po := range result.PaymentOrders {
-		protoOrders = append(protoOrders, toProto(po))
-	}
-
 	return &pb.ListPaymentOrdersResponse{
-		PaymentOrders: protoOrders,
+		PaymentOrders: lo.Map(result.PaymentOrders, func(po *domain.PaymentOrder, _ int) *pb.PaymentOrder {
+			return toProto(po)
+		}),
 		Pagination: &commonpb.PaginationResponse{
 			NextPageToken: result.NextCursor,
 			TotalCount:    result.TotalCount,

internal/platform/observability/grpc.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 	"sync"
 
+	"github.com/samber/lo"
 	"go.opentelemetry.io/otel/attribute"
 	"go.opentelemetry.io/otel/codes"
 	"go.opentelemetry.io/otel/propagation"
@@ -459,9 +460,5 @@ func (mc metadataCarrier) Set(key string, value string) {
 
 // Keys lists the keys stored in this carrier.
 func (mc metadataCarrier) Keys() []string {
-	keys := make([]string, 0, len(mc))
-	for k := range mc {
-		keys = append(keys, k)
-	}
-	return keys
+	return lo.Keys(mc)
 }

internal/position-keeping/repository/postgres_repository.go

@@ -14,6 +14,7 @@ import (
 	"github.com/jackc/pgx/v5/pgconn"
 	"github.com/jackc/pgx/v5/pgxpool"
 	"github.com/meridianhub/meridian/internal/position-keeping/domain"
+	"github.com/samber/lo"
 	"github.com/shopspring/decimal"
 )
 
@@ -357,12 +358,10 @@ func (r *PostgresRepository) Update(ctx context.Context, log *domain.FinancialPo
 	}
 
 	// Filter for only new audit entries
-	newAuditEntries := make([]*domain.AuditTrailEntry, 0)
-	for _, entry := range log.AuditTrail {
-		if _, exists := existingAuditIDs[entry.AuditID]; !exists {
-			newAuditEntries = append(newAuditEntries, entry)
-		}
-	}
+	newAuditEntries := lo.Filter(log.AuditTrail, func(entry *domain.AuditTrailEntry, _ int) bool {
+		_, exists := existingAuditIDs[entry.AuditID]
+		return !exists
+	})
 
 	// Insert only new audit entries
 	if len(newAuditEntries) > 0 {