Overview of the Nix Language: Difference between revisions

← Older edit Newer edit →

VisualWikitext

@@ Line 96: / Line 96: @@
 If you apply one argument <code>f 3</code>only, a partial function <code>y: 3*y</code>is returned.
-If the <code>argument</code> is a set like <code>{ hello="world"; a="text"; }</code>, you can pattern-match/destructure it:
+==== Destructuring ====
+In nix it happens that sets are given as arguments to functions. Say that we declare a function which returns the concatination of attributes a and b of a set like:
+<code>
+concat_a_and_b =  set: set.a + set.b
+ concat_a_and_b { a="hello"; b="world"; }
+"helloworld"
+</code>
+It is then possible to destructure the argument <code>set</code> and pick out the attributes that we are interested in, in the function declaration, resulting in a tidier function:
+<code>
+ concat_a_and_b = {a, b}: a + b
+concat_a_and_b { a="hello "; b="world"; }
+"hello world"
+</code>
+==== Default argument ====
+It is also possible to assign default values to be used in a function if the caller emits one, but only as part of a set.
+<code>
+add_a_b = { a ? 1, b ? 2 }: a + b
+add_a_b {}
+add_a_b {a=5;}
+</code>
+==== Accepting unexpected attributes in argument set ====
+If you want your function to still run without error if the user provides a set with more attributes than you expected it to have you can use the ellipses.
+<code>
+add_a_b = { a, b }: a + b
+add_a_b { a=5; b=2; c=10; }
+error: anonymous function at (string):1:2 called with unexpected argument 'c', at (string):1:1
+add_a_b = { a, b, ... }: a + b
+add_a_b { a=5; b=2; c=10; }
+</code>
+You can also store away the extra arguments in a name of your chosing using the <code>@</code> pattern.
+<code>
+add_a_b = rest@{ a, b, ... }: a + b + rest.c
+ add_a_b { a=5; b=2; c=10; }
+</code>
-# <code>{ hello, y ? "foo", z ? "bar", ... }: z + y + hello</code> a set pattern, which can also set defaults. ellipse means that the function may receive extra arguments.
-# <code>args@{ hello, y, z, ... }: z + y + hello + args.a</code> an <code>@</code> pattern which can be used to match extra arguments and store them in the <code>args</code> set.
 === Operators ===