gen-types

gen-types is a pure, nixpkgs.lib-free structural type checker for Nix — the checking half of a pure-Nix module system. A type is a predicate boundary: verify a value and get back null on success or an error string on failure.

The problem it owns: lib.types couples checking and merging into one mechanism, and a pure-gen module system needs the two split so gen-merge can own merging without needing nixpkgs. gen-types is the half that answers “is this value of this type,” and nothing else.

It depends on gen-identity and gen-prelude. Two roster members build on it: gen-merge, which hands gen-types leaf checkers off to evalModuleTree, and gen-settings.

Take it alone

{
  inputs.gen-types.url = "github:sini/gen-types";
}

Every constructor returns a record:

{
  name;    # full structural name, e.g. "listOf<int>"
  verify;  # value -> null | errString      (null = ok)
  check;   # v: v2: throws verify's error on failure, else returns v2
  __name;  # base name with polymorphic metadata stripped ("listOf")
  __mint;  # tagged identity regime
  __id;    # the accessor for a consumer demanding an identity
}

t.int.verify 5              # => null
t.int.verify "x"            # => "expected type 'int' but value \"x\" is of type 'string'"
t.int.check 5 5             # => 5        (validate-and-pass-through)