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lib/attrsets: add genAttrs' #270127

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lib/attrsets: add genAttrs' #270127

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lib/tests: Add test for genAttrs
dali99 Nov 26, 2023
7bbba3f
lib/attrsets: add genAttrs'
dali99 Nov 26, 2023
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14 changes: 14 additions & 0 deletions lib/attrsets.nix
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Original file line number Diff line number Diff line change
Expand Up @@ -651,6 +651,20 @@ rec {
f:
listToAttrs (map (n: nameValuePair n (f n)) names);

/* Generate an attribute set by mapping over a list of items
Taking two functions, one to apply for the name and one to
apply for the value of the attribute set.

Example:
genAttrs' [ {a = 2; b.c = "a" } {a = 1; b.c = "b"} ]
(i: i.b.c) (i: i.a)
Comment on lines +659 to +660
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Don't overload a and b, e.g.:

Suggested change
genAttrs' [ {a = 2; b.c = "a" } {a = 1; b.c = "b"} ]
(i: i.b.c) (i: i.a)
genAttrs' [ {p = 2; q.r = "a" } {p = 1; q.r = "b"} ]
(i: i.q.r) (i: i.p)

=> { a = 2; b = 1; }

Type:
genAttrs' :: [ a ] -> (a -> String) -> (a -> Any) -> AttrSet
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A denotationally equivalent function would be

-genAttrs' :: [ a ] -> (a -> String) -> (a -> Any) -> AttrSet
+genAttrs' :: [ a ] -> (a -> { name :: String; value :: a}) -> AttrSet

However, operationally, the latter can use a let binding to share computation between the two attributes.
The prior can not share a variable that depends on the a.

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Yet another alternative is

genAttrs' :: [ a ] -> (a -> Attrs) -> Attrs

This more compact, more powerful, and you can still read that a name and value are computed.

genAttrs' [ {a = 2; b.c = "a" } {a = 1; b.c = "b"} ]
  (i: { ${i.b.c} = i.a; })

while also allowing filtering behavior:

genAttrs' [ {a = 2; b.c = "a" } {a = 1; b.c = "b"} ]
  (i: optionalAttrs (i.a > 1) { ${i.b.c} = i.a; })

or multiple keys per item

genAttrs' [ {a = 2; b.c = "a" } {a = 1; b.c = "b"} ]
  (i: {
    ${i.b.c} = i.a;
    "${i.b.c}-squared" = i.a * i.a;
  })

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Whatever genAttrs' ends up being, its documentation should refer to the other functions that are similar, and the other functions should refer back.
Some functions I'd think of

  • genAttrs
  • concatMapAttrs
  • listToAttrs

There's probably a more related function I'm not thinking of.

*/
genAttrs' = items: f: g:
listToAttrs (map (i: nameValuePair (f i) (g i)) items);

/* Check whether the argument is a derivation. Any set with
`{ type = "derivation"; }` counts as a derivation.
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11 changes: 11 additions & 0 deletions lib/tests/misc.nix
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Expand Up @@ -785,6 +785,17 @@ runTests {
};
};

testGenAttrs = {
expr = attrsets.genAttrs [ "foo" "bar" ] (name: "${name}123");
expected = { foo = "foo123"; bar = "bar123"; };
};
testGenAttrsPrime = {
expr = attrsets.genAttrs' [ { foo = "a"; bar = "b"; } { foo = "c"; bar = "d"; } ] (x: x.foo) (x: "${toString x.foo}${toString x.bar}");
expected = {
a = "ab";
c = "cd";
};
};

testMergeAttrsListExample1 = {
expr = attrsets.mergeAttrsList [ { a = 0; b = 1; } { c = 2; d = 3; } ];
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