build: return ErrorList type

Adds a very similar interface to go/scanner package for returning a list
of errors.

Updates #34

build/cli.go

@@ -14,6 +14,7 @@ import (
 // Config contains options for an avo main function.
 type Config struct {
 	ErrOut     io.Writer
+	MaxErrors  int // max errors to report; 0 means unlimited
 	CPUProfile io.WriteCloser
 	Passes     []pass.Interface
 }
@@ -33,11 +34,9 @@ func Main(cfg *Config, context *Context) int {
 		defer pprof.StopCPUProfile()
 	}
 
-	f, errs := context.Result()
-	if errs != nil {
-		for _, err := range errs {
-			diag.Println(err)
-		}
+	f, err := context.Result()
+	if err != nil {
+		LogError(diag, err, cfg.MaxErrors)
 		return 1
 	}
 
@@ -52,9 +51,10 @@ func Main(cfg *Config, context *Context) int {
 
 // Flags represents CLI flags for an avo program.
 type Flags struct {
-	errout   *outputValue
-	cpuprof  *outputValue
-	printers []*printerValue
+	errout    *outputValue
+	allerrors bool
+	cpuprof   *outputValue
+	printers  []*printerValue
 }
 
 // NewFlags initializes avo flags for the given FlagSet.
@@ -64,6 +64,8 @@ func NewFlags(fs *flag.FlagSet) *Flags {
 	f.errout = newOutputValue(os.Stderr)
 	fs.Var(f.errout, "log", "diagnostics output")
 
+	fs.BoolVar(&f.allerrors, "e", false, "no limit on number of errors reported")
+
 	f.cpuprof = newOutputValue(nil)
 	fs.Var(f.cpuprof, "cpuprofile", "write cpu profile to `file`")
 
@@ -88,11 +90,19 @@ func (f *Flags) Config() *Config {
 			passes = append(passes, p)
 		}
 	}
-	return &Config{
+
+	cfg := &Config{
 		ErrOut:     f.errout.w,
+		MaxErrors:  10,
 		CPUProfile: f.cpuprof.w,
 		Passes:     passes,
 	}
+
+	if f.allerrors {
+		cfg.MaxErrors = 0
+	}
+
+	return cfg
 }
 
 type outputValue struct {

build/context.go

@@ -19,7 +19,7 @@ type Context struct {
 	file     *avo.File
 	function *avo.Function
 	global   *avo.Global
-	errs     []error
+	errs     ErrorList
 	reg.Collection
 }
 
@@ -176,8 +176,7 @@ func (c *Context) activeglobal() *avo.Global {
 }
 
 func (c *Context) adderror(err error) {
-	e := exterr(err)
-	c.errs = append(c.errs, e)
+	c.errs.addext(err)
 }
 
 func (c *Context) adderrormessage(msg string) {
@@ -185,6 +184,6 @@ func (c *Context) adderrormessage(msg string) {
 }
 
 // Result returns the built file and any accumulated errors.
-func (c *Context) Result() (*avo.File, []error) {
-	return c.file, c.errs
+func (c *Context) Result() (*avo.File, error) {
+	return c.file, c.errs.Err()
 }

build/error.go

@@ -1,6 +1,9 @@
 package build
 
 import (
+	"fmt"
+	"log"
+
 	"github.com/mmcloughlin/avo/internal/stack"
 	"github.com/mmcloughlin/avo/src"
 )
@@ -28,3 +31,53 @@ func (e Error) Error() string {
 	}
 	return msg
 }
+
+// ErrorList is a collection of errors for a source file.
+type ErrorList []Error
+
+// Add appends an error to the list.
+func (e *ErrorList) Add(err Error) {
+	*e = append(*e, err)
+}
+
+// addext appends an error to the list, tagged with the
+func (e *ErrorList) addext(err error) {
+	e.Add(exterr(err))
+}
+
+// Err returns an error equivalent to this error list.
+// If the list is empty, Err returns nil.
+func (e ErrorList) Err() error {
+	if len(e) == 0 {
+		return nil
+	}
+	return e
+}
+
+// An ErrorList implements the error interface.
+func (e ErrorList) Error() string {
+	switch len(e) {
+	case 0:
+		return "no errors"
+	case 1:
+		return e[0].Error()
+	}
+	return fmt.Sprintf("%s (and %d more errors)", e[0], len(e)-1)
+}
+
+// LogError logs a list of errors, one error per line, if the err parameter is
+// an ErrorList. Otherwise it just logs the err string. Reports at most max
+// errors, or unlimited if max is 0.
+func LogError(l *log.Logger, err error, max int) {
+	if list, ok := err.(ErrorList); ok {
+		for i, e := range list {
+			l.Printf("%s\n", e)
+			if max > 0 && i == max {
+				l.Print("too many errors")
+				return
+			}
+		}
+	} else if err != nil {
+		l.Printf("%s\n", err)
+	}
+}

pass/reg_test.go

@@ -3,6 +3,7 @@ package pass_test
 import (
 	"testing"
 
+	"github.com/mmcloughlin/avo/internal/test"
 	"github.com/mmcloughlin/avo/reg"
 
 	"github.com/mmcloughlin/avo/pass"
@@ -57,11 +58,9 @@ func AssertRegistersMatchSet(t *testing.T, rs []reg.Register, s reg.Set) {
 }
 
 func ConstructLiveness(t *testing.T, ctx *build.Context) *avo.Function {
-	f, errs := ctx.Result()
-	if errs != nil {
-		for _, err := range errs {
-			t.Error(err)
-		}
+	f, err := ctx.Result()
+	if err != nil {
+		build.LogError(test.Logger(t), err, 0)
 		t.FailNow()
 	}