diff options
Diffstat (limited to 'nix')
| -rw-r--r-- | nix/tag/default.nix | 149 | ||||
| -rw-r--r-- | nix/tag/tests.nix | 88 |
2 files changed, 237 insertions, 0 deletions
diff --git a/nix/tag/default.nix b/nix/tag/default.nix new file mode 100644 index 000000000000..832305368f95 --- /dev/null +++ b/nix/tag/default.nix @@ -0,0 +1,149 @@ +{ depot, lib, ... }: +let + # Takes a tag, checks whether it is an attrset with one element, + # if so sets `isTag` to `true` and sets the name and value. + # If not, sets `isTag` to `false` and sets `errmsg`. + verifyTag = tag: + let cases = builtins.attrNames tag; + len = builtins.length cases; + in + if builtins.length cases == 1 + then let name = builtins.head cases; in { + isTag = true; + name = name; + val = tag.${name}; + errmsg = null; + } + else { + isTag = false; + errmsg = + ( "match: an instance of a sum is an attrset " + + "with exactly one element, yours had ${toString len}" + + ", namely: ${lib.generators.toPretty {} cases}" ); + name = null; + val = null; + }; + + # like `isTag`, but throws the error message if it is not a tag. + assertIsTag = tag: + let res = verifyTag tag; in + assert lib.assertMsg res.isTag res.errmsg; + { inherit (res) name val; }; + + + # Discriminator for values. + # Goes through a list of tagged predicates `{ <tag> = <pred>; }` + # and returns the value inside the tag + # for which the first predicate applies, `{ <tag> = v; }`. + # They can then later be matched on with `match`. + # + # `defTag` is the tag that is assigned if there is no match. + # + # Examples: + # discrDef "smol" [ + # { biggerFive = i: i > 5; } + # { negative = i: i < 0; } + # ] -100 + # => { negative = 100; } + # discrDef "smol" [ + # { biggerFive = i: i > 5; } + # { negative = i: i < 0; } + # ] 1 + # => { smol = 1; } + discrDef = defTag: fs: v: + let res = lib.findFirst + (t: t.val v) + null + (map assertIsTag fs); + in + if res == null + then { ${defTag} = v; } + else { ${res.name} = v; }; + + # Like `discrDef`, but fail if there is no match. + discr = fs: v: + let res = discrDef null fs v; in + assert lib.assertMsg (res != null) + "tag.discr: No predicate found that matches ${lib.generators.toPretty {} v}"; + res; + + # The canonical pattern matching primitive. + # A sum value is an attribute set with one element, + # whose key is the name of the variant and + # whose value is the content of the variant. + # `matcher` is an attribute set which enumerates + # all possible variants as keys and provides a function + # which handles each variant’s content. + # You should make an effort to return values of the same + # type in your matcher, or new sums. + # + # Example: + # let + # success = { res = 42; }; + # failure = { err = "no answer"; }; + # matcher = { + # res = i: i + 1; + # err = _: 0; + # }; + # in + # match success matcher == 43 + # && match failure matcher == 0; + # + match = sum: matcher: + let cases = builtins.attrNames sum; + in assert + let len = builtins.length cases; in + lib.assertMsg (len == 1) + ( "match: an instance of a sum is an attrset " + + "with exactly one element, yours had ${toString len}" + + ", namely: ${lib.generators.toPretty {} cases}" ); + let case = builtins.head cases; + in assert + lib.assertMsg (matcher ? ${case}) + ( "match: \"${case}\" is not a valid case of this sum, " + + "the matcher accepts: ${lib.generators.toPretty {} + (builtins.attrNames matcher)}" ); + matcher.${case} sum.${case}; + + # A `match` with the arguments flipped. + # “Lam” stands for “lambda”, because it can be used like the + # `\case` LambdaCase statement in Haskell, to create a curried + # “matcher” function ready to take a value. + # + # Example: + # lib.pipe { foo = 42; } [ + # (matchLam { + # foo = i: if i < 23 then { small = i; } else { big = i; }; + # bar = _: { small = 5; }; + # }) + # (matchLam { + # small = i: "yay it was small"; + # big = i: "whoo it was big!"; + # }) + # ] + # => "whoo it was big!"; + matchLam = matcher: sum: match sum matcher; + + tests = import ./tests.nix { + inherit + depot + lib + verifyTag + discr + discrDef + match + matchLam + ; + }; + +in { + inherit + verifyTag + assertIsTag + discr + discrDef + match + matchLam + tests + ; +} diff --git a/nix/tag/tests.nix b/nix/tag/tests.nix new file mode 100644 index 000000000000..8c9c73807472 --- /dev/null +++ b/nix/tag/tests.nix @@ -0,0 +1,88 @@ +{ depot, lib, verifyTag, discr, discrDef, match, matchLam }: + +let + inherit (depot.nix.runTestsuite) + runTestsuite + assertEq + it + ; + + isTag-test = it "checks whether something is a tag" [ + (assertEq "is Tag" + (verifyTag { foo = "bar"; }) + { + isTag = true; + name = "foo"; + val = "bar"; + errmsg = null; + }) + (assertEq "is not Tag" + (removeAttrs (verifyTag { foo = "bar"; baz = 42; }) ["errmsg"]) + { + isTag = false; + name = null; + val = null; + }) + ]; + + discr-test = it "can discr things" [ + (assertEq "id" + (discr [ + { a = lib.const true; } + ] "x") + { a = "x"; }) + (assertEq "bools here, ints there" + (discr [ + { bool = lib.isBool; } + { int = lib.isInt; } + ] 25) + { int = 25; }) + (assertEq "bools here, ints there 2" + (discr [ + { bool = lib.isBool; } + { int = lib.isInt; } + ] true) + { bool = true; }) + (assertEq "fallback to default" + (discrDef "def" [ + { bool = lib.isBool; } + { int = lib.isInt; } + ] "foo") + { def = "foo"; }) + ]; + + match-test = it "can match things" [ + (assertEq "match example" + (let + success = { res = 42; }; + failure = { err = "no answer"; }; + matcher = { + res = i: i + 1; + err = _: 0; + }; + in { + one = match success matcher; + two = match failure matcher; + }) + { one = 43; + two = 0; }) + (assertEq "matchLam & pipe" + (lib.pipe { foo = 42; } [ + (matchLam { + foo = i: if i < 23 then { small = i; } else { big = i; }; + bar = _: { small = 5; }; + }) + (matchLam { + small = i: "yay it was small"; + big = i: "whoo it was big!"; + }) + ]) + "whoo it was big!") + ]; + +in + runTestsuite "tag" [ + isTag-test + discr-test + match-test + ] |