give more information in StringIndexError (#36054)

Author: KristofferC

base/strings/basic.jl

@@ -133,7 +133,7 @@ julia> isvalid(str, 2)
 false
 
 julia> str[2]
-ERROR: StringIndexError("αβγdef", 2)
+ERROR: StringIndexError: invalid index [2], valid nearby indices [1]=>'α', [3]=>'β'
 Stacktrace:
 [...]
 ```

base/strings/string.jl

@@ -11,6 +11,19 @@ struct StringIndexError <: Exception
 end
 @noinline string_index_err(s::AbstractString, i::Integer) =
     throw(StringIndexError(s, Int(i)))
+function Base.showerror(io::IO, exc::StringIndexError)
+    s = exc.string
+    print(io, "StringIndexError: ", "invalid index [$(exc.index)]")
+    if firstindex(s) <= exc.index <= ncodeunits(s)
+        iprev = thisind(s, exc.index)
+        inext = nextind(s, iprev)
+        if inext <= ncodeunits(s)
+            print(io, ", valid nearby indices [$iprev]=>'$(s[iprev])', [$inext]=>'$(s[inext])'")
+        else
+            print(io, ", valid nearby index [$iprev]=>'$(s[iprev])'")
+        end
+    end
+end
 
 const ByteArray = Union{Vector{UInt8},Vector{Int8}}
 

doc/src/manual/strings.md

@@ -279,11 +279,12 @@ julia> s[1]
 '∀': Unicode U+2200 (category Sm: Symbol, math)
 
 julia> s[2]
-ERROR: StringIndexError("∀ x ∃ y", 2)
+ERROR: StringIndexError: invalid index [2], valid nearby indices [1]=>'∀', [4]=>' '
+Stacktrace:
 [...]
 
 julia> s[3]
-ERROR: StringIndexError("∀ x ∃ y", 3)
+ERROR: StringIndexError: invalid index [3], valid nearby indices [1]=>'∀', [4]=>' '
 Stacktrace:
 [...]
 
@@ -303,7 +304,7 @@ julia> s[end-1]
 ' ': ASCII/Unicode U+0020 (category Zs: Separator, space)
 
 julia> s[end-2]
-ERROR: StringIndexError("∀ x ∃ y", 9)
+ERROR: StringIndexError: invalid index [9], valid nearby indices [7]=>'∃', [10]=>' '
 Stacktrace:
 [...]
 
@@ -323,7 +324,7 @@ julia> s[1:1]
 "∀"
 
 julia> s[1:2]
-ERROR: StringIndexError("∀ x ∃ y", 2)
+ERROR: StringIndexError: invalid index [2], valid nearby indices [1]=>'∀', [4]=>' '
 Stacktrace:
 [...]
 

test/strings/basic.jl

@@ -1076,3 +1076,12 @@ let x = SubString("ab", 1, 1)
     @test y === x
     chop("ab") === chop.(["ab"])[1]
 end
+
+@testset "show StringIndexError" begin
+    str = "abcdefghκijklmno"
+    e = StringIndexError(str, 10)
+    @test sprint(showerror, e) == "StringIndexError: invalid index [10], valid nearby indices [9]=>'κ', [11]=>'i'"
+    str = "κ"
+    e = StringIndexError(str, 2)
+    @test sprint(showerror, e) == "StringIndexError: invalid index [2], valid nearby index [1]=>'κ'"
+end