fmt/Printf: document and tweak error messages produced for bad formats

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/2198044

diff --git a/src/pkg/fmt/doc.go b/src/pkg/fmt/doc.go
index 25184d1e45f7eaf3ff39c8537957a501db948ec1..06dc730089d5559be245ee3a9ee554d33173bbf7 100644 (file)
--- a/src/pkg/fmt/doc.go
+++ b/src/pkg/fmt/doc.go
@@ -69,8 +69,8 @@
        Regardless of the verb, if an operand is an interface value,
        the internal concrete value is used, not the interface itself.
        Thus:
-               var i interface{} = 23;
-               fmt.Printf("%v\n", i);
+               var i interface{} = 23
+               fmt.Printf("%v\n", i)
        will print 23.
 
        If an operand implements interface Formatter, that interface
@@ -85,6 +85,26 @@
        cast the value before recurring:
                func (x X) String() string { return Sprintf("%d", int(x)) }
 
+       Format errors:
+
+       If an invalid argument is given for a verb, such as providing
+       a string to %d, the generated string will contain a
+       description of the problem, as in these examples:
+
+               Wrong type or unknown verb: %!verb(type=value)
+                       Printf("%d", hi):          %!d(string=hi)
+               Too many arguments: %!(EXTRA type=value)
+                       Printf("hi", "guys"):      hi%!(EXTRA string=guys)
+               Too few arguments: %!verb(MISSING)
+                       Printf("hi%d"):            hi %!d(MISSING)
+               Non-int for width or precision: %!(BADWIDTH) or %!(BADPREC)
+                       Printf("%*s", 4.5, "hi"):  %!(BADWIDTH)hi
+                       Printf("%.*s", 4.5, "hi"): %!(BADPREC)hi
+
+       All errors begin with the string "%!" followed sometimes
+       by a single character (the verb) and end with a parenthesized
+       description.
+
        Scanning:
 
        An analogous set of functions scans formatted text to yield
@@ -97,7 +117,7 @@
        routines treat newlines as spaces.
 
        Scanf, Fscanf, and Sscanf parse the arguments according to a
-       format string, analogous to that of Printf.  For example, "%x"
+       format string, analogous to that of Printf.  For example, %x
        will scan an integer as a hexadecimal number, and %v will scan
        the default representation format for the value.
 

diff --git a/src/pkg/fmt/fmt_test.go b/src/pkg/fmt/fmt_test.go
index b8d15a7dc73da467cdb5cabb0db9d28008665f9d..c8775ba3f2cc30f407739c22d61861dd6d1d51e9 100644 (file)
--- a/src/pkg/fmt/fmt_test.go
+++ b/src/pkg/fmt/fmt_test.go
@@ -193,8 +193,8 @@ var fmttests = []fmtTest{
        fmtTest{"%+.3g", complex128(1 + 2i), "(+1+2i)"},
 
        // erroneous formats
-       fmtTest{"", 2, "?(extra int=2)"},
-       fmtTest{"%d", "hello", "%d(string=hello)"},
+       fmtTest{"", 2, "%!(EXTRA int=2)"},
+       fmtTest{"%d", "hello", "%!d(string=hello)"},
 
        // old test/fmt_test.go
        fmtTest{"%d", 1234, "1234"},
@@ -301,7 +301,7 @@ var fmttests = []fmtTest{
        fmtTest{"%s", I(23), `<23>`},
        fmtTest{"%q", I(23), `"<23>"`},
        fmtTest{"%x", I(23), `3c32333e`},
-       fmtTest{"%d", I(23), `%d(string=<23>)`},
+       fmtTest{"%d", I(23), `%!d(string=<23>)`},
 
        // go syntax
        fmtTest{"%#v", A{1, 2, "a", []int{1, 2}}, `fmt_test.A{i:1, j:0x2, s:"a", x:[]int{1, 2}}`},
@@ -321,7 +321,7 @@ var fmttests = []fmtTest{
 
        // renamings
        fmtTest{"%v", renamedBool(true), "true"},
-       fmtTest{"%d", renamedBool(true), "%d(fmt_test.renamedBool=true)"},
+       fmtTest{"%d", renamedBool(true), "%!d(fmt_test.renamedBool=true)"},
        fmtTest{"%o", renamedInt(8), "10"},
        fmtTest{"%d", renamedInt8(-9), "-9"},
        fmtTest{"%v", renamedInt16(10), "10"},
@@ -366,14 +366,14 @@ var fmttests = []fmtTest{
        fmtTest{"%p", make(chan int), "PTR"},
        fmtTest{"%p", make(map[int]int), "PTR"},
        fmtTest{"%p", make([]int, 1), "PTR"},
-       fmtTest{"%p", 27, "%p(int=27)"}, // not a pointer at all
+       fmtTest{"%p", 27, "%!p(int=27)"}, // not a pointer at all
 
        // erroneous things
-       fmtTest{"%d", "hello", "%d(string=hello)"},
-       fmtTest{"no args", "hello", "no args?(extra string=hello)"},
-       fmtTest{"%s", nil, "%s(<nil>)"},
+       fmtTest{"%d", "hello", "%!d(string=hello)"},
+       fmtTest{"no args", "hello", "no args%!(EXTRA string=hello)"},
+       fmtTest{"%s", nil, "%!s(<nil>)"},
        fmtTest{"%T", nil, "<nil>"},
-       fmtTest{"%-1", 100, "%1(int=100)"},
+       fmtTest{"%-1", 100, "%!1(int=100)"},
 }
 
 func TestSprintf(t *testing.T) {
@@ -622,12 +622,12 @@ var startests = []starTest{
        starTest{"%-*d", args(4, 42), "42  "},
 
        // erroneous
-       starTest{"%*d", args(nil, 42), "%(badwidth)42"},
-       starTest{"%.*d", args(nil, 42), "%(badprec)42"},
-       starTest{"%*d", args(5, "foo"), "%d(string=  foo)"},
+       starTest{"%*d", args(nil, 42), "%!(BADWIDTH)42"},
+       starTest{"%.*d", args(nil, 42), "%!(BADPREC)42"},
+       starTest{"%*d", args(5, "foo"), "%!d(string=  foo)"},
        starTest{"%*% %d", args(20, 5), "% 5"},
-       starTest{"%*", args(4), "%(badwidth)%*(int=4)"},
-       starTest{"%*d", args(int32(4), 42), "%(badwidth)42"},
+       starTest{"%*", args(4), "%!(BADWIDTH)%!*(int=4)"},
+       starTest{"%*d", args(int32(4), 42), "%!(BADWIDTH)42"},
 }
 
 // TODO: there's no conversion from []T to ...T, but we can fake it.  These

diff --git a/src/pkg/fmt/print.go b/src/pkg/fmt/print.go
index 8585c2dcaf40576073c749943ea2f3191b2eec3d..33095627dcd9b408e170feb0826579c7fb6ccffa 100644 (file)
--- a/src/pkg/fmt/print.go
+++ b/src/pkg/fmt/print.go
@@ -20,12 +20,12 @@ var (
        nilParenBytes   = []byte("(nil)")
        nilBytes        = []byte("nil")
        mapBytes        = []byte("map[")
-       missingBytes    = []byte("missing")
-       extraBytes      = []byte("?(extra ")
+       missingBytes    = []byte("(MISSING)")
+       extraBytes      = []byte("%!(EXTRA ")
        irparenBytes    = []byte("i)")
        bytesBytes      = []byte("[]byte{")
-       widthBytes      = []byte("%(badwidth)")
-       precBytes       = []byte("%(badprec)")
+       widthBytes      = []byte("%!(BADWIDTH)")
+       precBytes       = []byte("%!(BADPREC)")
 )
 
 // State represents the printer state passed to custom formatters.
@@ -266,6 +266,7 @@ func (p *pp) unknownType(v interface{}) {
 
 func (p *pp) badVerb(verb int, val interface{}) {
        p.add('%')
+       p.add('!')
        p.add(verb)
        p.add('(')
        if val == nil {