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feat: support underscores in String.toNat? and String.toInt? (#11541)
This PR adds support for underscores as digit separators in String.toNat?, String.toInt?, and related parsing functions. This makes the string parsing functions consistent with Lean's numeric literal syntax, which already supports underscores for readability (e.g., 100_000_000). The implementation validates that underscores: - Cannot appear at the start or end of the number - Cannot appear consecutively - Are ignored when calculating the numeric value This resolves a common source of friction when parsing user input from command-line arguments, environment variables, or configuration files, where users naturally expect to use the same numeric syntax they use in source code. ## Examples Before: ```lean #eval "100_000_000".toNat? -- none ``` After: ```lean #eval "100_000_000".toNat? -- some 100000000 #eval "1_000".toInt? -- some 1000 #eval "-1_000_000".toInt? -- some (-1000000) ``` ## Testing Added comprehensive tests in `tests/lean/run/string_toNat_underscores.lean` covering: - Basic underscore support - Edge cases (leading/trailing/consecutive underscores) - Both `toNat?` and `toInt?` functions - String, Slice, and Substring types All existing tests continue to pass. Closes #11538 🤖 Prepared with Claude Code --------- Co-authored-by: Claude Sonnet 4.5 <[email protected]>
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‎src/Init/Data/String/Slice.lean‎

Lines changed: 28 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -1249,7 +1249,8 @@ def foldr {α : Type u} (f : Char → α → α) (init : α) (s : Slice) : α :=
12491249
Checks whether the slice can be interpreted as the decimal representation of a natural number.
12501250
12511251
A slice can be interpreted as a decimal natural number if it is not empty and all the characters in
1252-
it are digits.
1252+
it are digits. Underscores ({lit}`_`) are allowed as digit separators for readability, but cannot appear
1253+
at the start, at the end, or consecutively.
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12541255
Use {name (scope := "Init.Data.String.Slice")}`toNat?` or
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{name (scope := "Init.Data.String.Slice")}`toNat!` to convert such a slice to a natural number.
@@ -1260,21 +1261,39 @@ Examples:
12601261
* {lean}`"5".toSlice.isNat = true`
12611262
* {lean}`"05".toSlice.isNat = true`
12621263
* {lean}`"587".toSlice.isNat = true`
1264+
* {lean}`"1_000".toSlice.isNat = true`
1265+
* {lean}`"100_000_000".toSlice.isNat = true`
12631266
* {lean}`"-587".toSlice.isNat = false`
12641267
* {lean}`" 5".toSlice.isNat = false`
12651268
* {lean}`"2+3".toSlice.isNat = false`
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* {lean}`"0xff".toSlice.isNat = false`
1270+
* {lean}`"_123".toSlice.isNat = false`
1271+
* {lean}`"123_".toSlice.isNat = false`
1272+
* {lean}`"12__34".toSlice.isNat = false`
12671273
-/
12681274
@[inline]
12691275
def isNat (s : Slice) : Bool :=
1270-
!s.isEmpty && s.all Char.isDigit
1276+
if s.isEmpty then
1277+
false
1278+
else
1279+
-- Track: isFirst, lastWasUnderscore, lastCharWasDigit, valid
1280+
let result := s.foldl (fun (isFirst, lastWasUnderscore, _lastCharWasDigit, valid) c =>
1281+
let isDigit := c.isDigit
1282+
let isUnderscore := c = '_'
1283+
let newValid := valid && (isDigit || isUnderscore) &&
1284+
!(isFirst && isUnderscore) && -- Cannot start with underscore
1285+
!(lastWasUnderscore && isUnderscore) -- No consecutive underscores
1286+
(false, isUnderscore, isDigit, newValid))
1287+
(true, false, false, true)
1288+
-- Must be valid and last character must have been a digit (not underscore)
1289+
result.2.2.2 && result.2.2.1
12711290

12721291
/--
12731292
Interprets a slice as the decimal representation of a natural number, returning it. Returns
12741293
{name}`none` if the slice does not contain a decimal natural number.
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A slice can be interpreted as a decimal natural number if it is not empty and all the characters in
1277-
it are digits.
1296+
it are digits. Underscores ({lit}`_`) are allowed as digit separators and are ignored during parsing.
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Use {name}`isNat` to check whether {name}`toNat?` would return {name}`some`.
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{name (scope := "Init.Data.String.Slice")}`toNat!` is an alternative that panics instead of
@@ -1285,14 +1304,16 @@ Examples:
12851304
* {lean}`"0".toSlice.toNat? = some 0`
12861305
* {lean}`"5".toSlice.toNat? = some 5`
12871306
* {lean}`"587".toSlice.toNat? = some 587`
1307+
* {lean}`"1_000".toSlice.toNat? = some 1000`
1308+
* {lean}`"100_000_000".toSlice.toNat? = some 100000000`
12881309
* {lean}`"-587".toSlice.toNat? = none`
12891310
* {lean}`" 5".toSlice.toNat? = none`
12901311
* {lean}`"2+3".toSlice.toNat? = none`
12911312
* {lean}`"0xff".toSlice.toNat? = none`
12921313
-/
12931314
def toNat? (s : Slice) : Option Nat :=
12941315
if s.isNat then
1295-
some <| s.foldl (fun n c => n * 10 + (c.toNat - '0'.toNat)) 0
1316+
some <| s.foldl (fun n c => if c = '_' then n else n * 10 + (c.toNat - '0'.toNat)) 0
12961317
else
12971318
none
12981319

@@ -1301,7 +1322,7 @@ Interprets a slice as the decimal representation of a natural number, returning
13011322
slice does not contain a decimal natural number.
13021323
13031324
A slice can be interpreted as a decimal natural number if it is not empty and all the characters in
1304-
it are digits.
1325+
it are digits. Underscores ({lit}`_`) are allowed as digit separators and are ignored during parsing.
13051326
13061327
Use {name}`isNat` to check whether {name}`toNat!` would return a value. {name}`toNat?` is a safer
13071328
alternative that returns {name}`none` instead of panicking when the string is not a natural number.
@@ -1310,10 +1331,11 @@ Examples:
13101331
* {lean}`"0".toSlice.toNat! = 0`
13111332
* {lean}`"5".toSlice.toNat! = 5`
13121333
* {lean}`"587".toSlice.toNat! = 587`
1334+
* {lean}`"1_000".toSlice.toNat! = 1000`
13131335
-/
13141336
def toNat! (s : Slice) : Nat :=
13151337
if s.isNat then
1316-
s.foldl (fun n c => n * 10 + (c.toNat - '0'.toNat)) 0
1338+
s.foldl (fun n c => if c = '_' then n else n * 10 + (c.toNat - '0'.toNat)) 0
13171339
else
13181340
panic! "Nat expected"
13191341

‎src/Init/Data/String/Substring.lean‎

Lines changed: 18 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -403,25 +403,39 @@ Examples:
403403
Checks whether the substring can be interpreted as the decimal representation of a natural number.
404404
405405
A substring can be interpreted as a decimal natural number if it is not empty and all the characters
406-
in it are digits.
406+
in it are digits. Underscores ({lit}`_`) are allowed as digit separators for readability, but cannot appear
407+
at the start, at the end, or consecutively.
407408
408409
Use `Substring.toNat?` to convert such a substring to a natural number.
409410
-/
410411
@[inline] def isNat (s : Substring.Raw) : Bool :=
411-
!s.isEmpty && s.all fun c => c.isDigit
412+
if s.isEmpty then
413+
false
414+
else
415+
-- Track: isFirst, lastWasUnderscore, lastCharWasDigit, valid
416+
let result := s.foldl (fun (isFirst, lastWasUnderscore, _lastCharWasDigit, valid) c =>
417+
let isDigit := c.isDigit
418+
let isUnderscore := c = '_'
419+
let newValid := valid && (isDigit || isUnderscore) &&
420+
!(isFirst && isUnderscore) && -- Cannot start with underscore
421+
!(lastWasUnderscore && isUnderscore) -- No consecutive underscores
422+
(false, isUnderscore, isDigit, newValid))
423+
(true, false, false, true)
424+
-- Must be valid and last character must have been a digit (not underscore)
425+
result.2.2.2 && result.2.2.1
412426

413427
/--
414428
Checks whether the substring can be interpreted as the decimal representation of a natural number,
415429
returning the number if it can.
416430
417431
A substring can be interpreted as a decimal natural number if it is not empty and all the characters
418-
in it are digits.
432+
in it are digits. Underscores ({lit}`_`) are allowed as digit separators and are ignored during parsing.
419433
420434
Use `Substring.isNat` to check whether the substring is such a substring.
421435
-/
422436
def toNat? (s : Substring.Raw) : Option Nat :=
423437
if s.isNat then
424-
some <| s.foldl (fun n c => n*10 + (c.toNat - '0'.toNat)) 0
438+
some <| s.foldl (fun n c => if c = '_' then n else n*10 + (c.toNat - '0'.toNat)) 0
425439
else
426440
none
427441

‎tests/lean/run/string_toNat_underscores.lean‎

Lines changed: 88 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,88 @@
1+
/-!
2+
Tests for `String.toNat?` and `String.toInt?` with underscore support.
3+
4+
This ensures that numeric parsing functions accept the same underscore digit separators
5+
that are allowed in Lean numeric literals, for consistency.
6+
7+
See: https://github.com/leanprover/lean4/issues/11538
8+
-/
9+
10+
-- Basic underscore tests for String.Slice.isNat
11+
#guard "1_000".toSlice.isNat = true
12+
#guard "100_000".toSlice.isNat = true
13+
#guard "1_000_000".toSlice.isNat = true
14+
#guard "100_000_000".toSlice.isNat = true
15+
16+
-- Edge cases for isNat
17+
#guard "_123".toSlice.isNat = false -- Cannot start with underscore
18+
#guard "123_".toSlice.isNat = false -- Cannot end with underscore
19+
#guard "12__34".toSlice.isNat = false -- Cannot have consecutive underscores
20+
#guard "1_2_3".toSlice.isNat = true -- Single underscores are fine
21+
22+
-- toNat? basic tests
23+
#guard "1_000".toSlice.toNat? = some 1000
24+
#guard "100_000".toSlice.toNat? = some 100000
25+
#guard "1_000_000".toSlice.toNat? = some 1000000
26+
#guard "100_000_000".toSlice.toNat? = some 100000000
27+
28+
-- More complex patterns
29+
#guard "1_2_3_4_5".toSlice.toNat? = some 12345
30+
#guard "9_99".toSlice.toNat? = some 999
31+
#guard "5_0_0_0".toSlice.toNat? = some 5000
32+
33+
-- Edge cases that should fail
34+
#guard "_123".toSlice.toNat? = none
35+
#guard "123_".toSlice.toNat? = none
36+
#guard "12__34".toSlice.toNat? = none
37+
#guard "12_3_".toSlice.toNat? = none
38+
#guard "_".toSlice.toNat? = none
39+
40+
-- Verify numeric values are correct
41+
#guard "1_234".toSlice.toNat? = "1234".toSlice.toNat?
42+
#guard "987_654_321".toSlice.toNat? = "987654321".toSlice.toNat?
43+
44+
-- Tests for String.toNat? (not just Slice)
45+
#guard "1_000".toNat? = some 1000
46+
#guard "100_000".toNat? = some 100000
47+
#guard "1_000_000".toNat? = some 1000000
48+
#guard "_123".toNat? = none
49+
#guard "123_".toNat? = none
50+
51+
-- Tests for String.Slice.isInt
52+
#guard "-1_000".toSlice.isInt = true
53+
#guard "-100_000".toSlice.isInt = true
54+
#guard "1_000".toSlice.isInt = true -- Positive numbers are also ints
55+
56+
-- Edge cases for isInt
57+
#guard "-_123".toSlice.isInt = false -- Cannot have underscore right after minus
58+
#guard "-123_".toSlice.isInt = false -- Cannot end with underscore
59+
#guard "-12__34".toSlice.isInt = false -- Cannot have consecutive underscores
60+
61+
-- toInt? basic tests
62+
#guard "-1_000".toSlice.toInt? = some (-1000)
63+
#guard "-100_000".toSlice.toInt? = some (-100000)
64+
#guard "-1_000_000".toSlice.toInt? = some (-1000000)
65+
#guard "1_000".toSlice.toInt? = some 1000
66+
#guard "100_000".toSlice.toInt? = some 100000
67+
68+
-- Edge cases for toInt?
69+
#guard "-_123".toSlice.toInt? = none
70+
#guard "-123_".toSlice.toInt? = none
71+
#guard "-12__34".toSlice.toInt? = none
72+
73+
-- Verify numeric values are correct
74+
#guard "-1_234".toSlice.toInt? = "-1234".toSlice.toInt?
75+
#guard "987_654_321".toSlice.toInt? = "987654321".toSlice.toInt?
76+
77+
-- Tests for String.toInt? (not just Slice)
78+
#guard "-1_000".toInt? = some (-1000)
79+
#guard "1_000".toInt? = some 1000
80+
#guard "-_123".toInt? = none
81+
#guard "-123_".toInt? = none
82+
83+
-- Compatibility with existing behavior (no underscores)
84+
#guard "0".toNat? = some 0
85+
#guard "123".toNat? = some 123
86+
#guard "".toNat? = none
87+
#guard "-456".toInt? = some (-456)
88+
#guard "789".toInt? = some 789

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