Nix (language): Difference between revisions
imported>Sitwon m Correcting the grammar of the sentence by removing an extra occurrence of 'into'. |
imported>Gabriel-doriath-dohler No edit summary |
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==== <code>with</code> statement ==== | ==== <code>with</code> statement ==== | ||
The [https://nixos.org/manual/nix/stable/#idm140737321903120 <code>with</code> statement] introduces an attrset's value contents into the lexical scope of the expression which follows. This means that it brings all keys within that set | The [https://nixos.org/manual/nix/stable/#idm140737321903120 <code>with</code> statement] introduces an attrset's value contents into the lexical scope of the expression which follows. This means that it brings all keys within that set into scope in that expression. So, you don't need to use the dot notation. If a key is already in the outer scope, it is only shadowed when it was introduced by a <code>with</code> statement. | ||
Example: | Example: | ||
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</syntaxHighlight> | </syntaxHighlight> | ||
Note that (perhaps surprisingly) <code>with</code> '''does not shadow''' | Note that (perhaps surprisingly) <code>with</code> '''does not shadow''' variables from outer scope which were bound in a <code>let</code> statement or a lambda abstraction. For example: | ||
<syntaxHighlight lang=nix> | <syntaxHighlight lang=nix> | ||
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This is because <code>a</code> was already defined in the scope outside the use of <code>with</code>, and <code>with</code> does ''not'' override it. The outer value takes precedence. (It is suspected by the author of this wiki section that the reason for this non-shadowing logic is lexical code stability: In the common usages of <code>with</code> as shown below, the contents of attrsets given to the statement are often large, community-maintained, and frequently updated. If e.g. <code>let myValue = ...; with lib; doSomethingWith myValue</code> shadowed the outer <code>myValue</code> bindings, the people maintaining `lib` could accidentally break this code by adding <code>lib.myValue</code>.) | This is because <code>a</code> was already defined in the scope outside the use of <code>with</code>, and <code>with</code> does ''not'' override it. The outer value takes precedence. (It is suspected by the author of this wiki section that the reason for this non-shadowing logic is lexical code stability: In the common usages of <code>with</code> as shown below, the contents of attrsets given to the statement are often large, community-maintained, and frequently updated. If e.g. <code>let myValue = ...; with lib; doSomethingWith myValue</code> shadowed the outer <code>myValue</code> bindings, the people maintaining `lib` could accidentally break this code by adding <code>lib.myValue</code>.) | ||
Note that variables are shadowed when (and only when) they were introduced by a <code>with</code> statement. For example: | |||
<syntaxHighlight lang=nix> | |||
nix-repl> with { x = "outer"; }; with { x = "inner"; }; x | |||
"inner" | |||
</syntaxHighlight> | |||
Common usages are: | Common usages are: | ||