89 lines
2.8 KiB
Nix
89 lines
2.8 KiB
Nix
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# Provides a "type-system" for Nix that provides various primitive &
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# polymorphic types as well as the ability to define & check records.
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#
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# All types (should) compose as expected.
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#
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# TODO:
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# - error messages for type-checks of map/list elements are bad
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# - enums?
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{ toPretty ? ((import <nixpkgs> {}).lib.generators.toPretty) }:
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# Checks are simply functions of the type `a -> bool` which check
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# whether `a` conforms to the specification.
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with builtins; let
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# Internal utilities:
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typeError = type: val:
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throw "Expected type '${type}', but value '${toPretty {} val}' is of type '${typeOf val}'";
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typedef = name: check: {
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inherit name check;
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__functor = self: value:
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if check value then value
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else typeError name value;
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};
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poly = n: c: { "${n}" = t: typedef "${n}<${t.name}>" (c t); };
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poly2 = n: c: {
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"${n}" = t1: t2: typedef "${n}<${t1.name},${t2.name}>" (c t1 t2);
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};
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typeSet = foldl' (s: t: s // (if t ? "name" then { "${t.name}" = t; } else t)) {};
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# Struct checker implementation
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#
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# Checks performed:
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# 1. All existing fields match their types
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# 2. No non-optional fields are missing.
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# 3. No unexpected fields are in the struct.
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#
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# Anonymous structs are supported (e.g. for nesting) by omitting the
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# name.
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checkField = def: value: current: field:
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let
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fieldVal = if hasAttr field value then value."${field}" else null;
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type = def."${field}";
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checked = type.check fieldVal;
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in if checked then (current && true)
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else (throw "Field ${field} is of type ${typeOf fieldVal}, but expected ${type.name}");
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checkExtraneous = name: def: present:
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if (length present) == 0 then true
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else if (hasAttr (head present) def)
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then checkExtraneous name def (tail present)
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else (throw "Found unexpected field '${head present}' in struct '${name}'");
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struct' = name: def: {
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inherit name def;
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check = value:
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let
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fieldMatch = foldl' (checkField def value) true (attrNames def);
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noExtras = checkExtraneous name def (attrNames value);
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in (isAttrs value && fieldMatch && noExtras);
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__functor = self: value: if self.check value
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then value
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else (throw "Expected '${self.name}'-struct, but ${toPretty value} is of type ${typeOf value}");
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};
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struct = arg:
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if isString arg then (struct' arg)
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else (struct' "anonymous" arg);
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in (typeSet [
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# Primitive types
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(typedef "any" (_: true))
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(typedef "int" isInt)
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(typedef "bool" isBool)
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(typedef "float" isFloat)
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(typedef "string" isString)
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# Polymorphic types
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(poly "option" (t: v: (isNull v) || t.check v))
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(poly "list" (t: v: isList v && (foldl' (s: e: s && (t.check e)) true v)))
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(poly "attrs" (t: v:
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isAttrs v && (foldl' (s: e: s && (t.check e)) true (attrValues v))
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))
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(poly2 "either" (t1: t2: v: t1.check v || t2.check v))
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]) // { inherit struct; }
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