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Constrain the length of a string
jpluscplusm
commented cue
validation
cue-lang/cue#2564
link from content to Issue
cue-lang/docs-and-content#114
Commented CUE explainer
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This [Commented CUE]({{< relref "docs/howto#commented-cue-guides" >}}) demonstrates how to make sure a string has a minimum length, or a maximum length, or both.

This CUE shows string fields being constrained by their lengths as expressed in bytes, and also in runes, as introduced by the Go programming language.

{{< code-tabs >}} {{< code-tab name="example.cue" language="cue" area="top" >}} package example

import "strings"

// 2 examples that use only single-byte, ASCII characters ascii_constrain_runes: "only ASCII characters" // 21 runes composed of 21 bytes // enforce that the field contains between 2 and 8 runes ascii_constrain_runes: strings.MinRunes(2) & strings.MaxRunes(8)

ascii_constrain_bytes: { // set up an alias to allow the following CUE to refer to it let content = "only ASCII characters" // 21 runes composed of 21 bytes

// this embedded scalar ultimately sets the value of the containing field,
// "ascii_constrain_bytes"
content

// enforce that "content" contains between 50 and 100 bytes
#bytes: len(content) & >=50 & <=100

}

// 2 examples that use multi-byte, South Korean Hangul characters multibyte_constrain_runes: "한글" // 2 runes composed of 6 bytes // enforce that the field contains 5 or fewer runes multibyte_constrain_runes: strings.MaxRunes(5)

multibyte_constrain_bytes: { // set up an alias to allow the following CUE to refer to it let content = "한글" // 2 runes composed of 6 bytes

// this embedded scalar ultimately sets the value of the containing field,
// "multibyte_constrain_bytes"
content

// enforce that "content" contains 5 or fewer bytes
#bytes: len(content) & <=5

}

// 2 examples that use multi-byte, emoji characters emoji_constrain_runes: "😄🥵🙃🥶" // 4 runes composed of 16 bytes // enforce that the field contains 10 or more runes emoji_constrain_runes: strings.MinRunes(10)

emoji_constrain_bytes: { // set up an alias to allow the following CUE to refer to it let content = "😄🥵🙃🥶" // 4 runes composed of 16 bytes

// this embedded scalar ultimately sets the value of the containing field,
// "emoji_constrain_bytes"
content

// enforce that "content" contains 10 or more bytes
#bytes: len(content) & >=10

} {{< /code-tab >}} {{< code-tab name="TERMINAL" language="err" type="terminal" area="bottom" >}} $ cue vet .:example ascii_constrain_runes: invalid value "only ASCII characters" (does not satisfy strings.MaxRunes(8)): ./example.cue:8:46 ./example.cue:6:24 ./example.cue:8:24 ./example.cue:8:63 ascii_constrain_bytes.#bytes: invalid value 21 (out of bound >=50): ./example.cue:19:25 ./example.cue:19:10 multibyte_constrain_bytes.#bytes: invalid value 6 (out of bound <=5): ./example.cue:36:25 ./example.cue:36:10 emoji_constrain_runes: invalid value "😄🥵🙃🥶" (does not satisfy strings.MinRunes(10)): ./example.cue:42:24 ./example.cue:40:24 ./example.cue:42:41 {{< /code-tab >}} {{< /code-tabs >}}

{{< info >}} The len()-based examples use an embedded scalar value with an alias to permit their fields to be of type string whilst also allowing their byte lengths to be constrained.

This pattern will be obsoleted by the must() directive, which is tracked in {{< issue 575 />}}. {{< /info >}}