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document the behavior of negative substring len

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Change-Id: I0777580a3aa374400586b76afdc74da5b5f6645a
This commit is contained in:
Charles Hall
2025-02-16 15:18:19 -08:00
parent 5b98b75f74
commit 93a4f0e82d
1 changed files with 10 additions and 4 deletions
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lix/libexpr/builtins/substring.md
+10 -4
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@@ -5,12 +5,18 @@ args: [start, len, s]
Return the substring of *s* from character position *start*
(zero-based) up to but not including *start + len*. If *start* is
greater than the length of the string, an empty string is returned,
and if *start + len* lies beyond the end of the string, only the
substring up to the end of the string is returned. *start* must be
non-negative. For example,
and if *start + len* lies beyond the end of the string or *len*
is negative, only the substring up to the end of the string is
returned. *start* must be non-negative. For example,
```nix
builtins.substring 0 3 "nixos"
```
evaluates to `"nix"`.
evaluates to `"nix"`, and
```nix
builtins.substring 3 (-1) "nixos"
```
evaluates to `"os"`.