intertype

InterType

A JavaScript type checker with helpers to implement own types and do object shape validation.

Table of Contents generated with DocToc

• Concepts
• Usage
• Declaring New Types
• Typed Value Casting
• Checks
  • Concatenating Checks
• Formal Definition of the Type Concept
• immediate and nowait
• To Do

Concepts

• what is a type

• fundamental types vs. domain types

• isa

• validate

• type_of

• types_of

• Types are defined using an ordered set of (one or more) named boolean test functions known as 'aspects'. In order for a value x to be of type T, all aspects—when called in their defined order with x (and possibly other arguments, see below)—have to return true. Aspect satisfication tests are done in a lazy fashion, so that no tests are performed after one has failed. Likewise for type validation, the difference being that the first failing aspect will cause an error to thrown that quotes the aspect's name.

• Types may be parametrized. For example, there's a 'partial' type multiple_of which needs a module (a number to be a multiple of) as extra parameters; thus, we can test isa.multiple_of 121, 11.

• In InterType, a 'type' is, on the one hand, essentially an ordered set of aspects; on the other hand, since within the context of a given InterType instance, each type corresponds to exactly one type name (a nonempty text), a 'type' can be identified with a string. Thus, the type of, say, [] is 'list' (i.e. the string that spells its name).

  Conversely, any list of functions that 1) can be called with a value as first arguments (possibly plus a number of extra parameters), that 2) never throws an error and 3) always returns a Boolean value can be regarded as a list of aspects, hence defining a (possibly empty) set of values.

Usage

[WIP]

One usage pattern for InterType is to make it so that one (sub-) project gets a module—call it types—that is dedicated to type declarations; requireing that types module then makes type checking and type validation methods available. Say we have:

# in module `types.coffee`

# instantiate InterType instance, export its methods to `module.exports` in one go:
intertype = new ( require 'intertype' ) module.exports

# now you can call methods of InterType instance as *module* methods:
@declare 'mytype', ( x ) -> ( @isa number ) and ( x > 12 ) and ( x <= 42 )

In another module:

# now use the declared types:
{ isa, type_of, validate, } = require './types'

console.log isa.integer     100   # true
console.log isa.mytype      20    # true
console.log isa.mytype      100   # false
console.log type_of         20    # 'number'
console.log validate.mytype 20    # true
console.log validate.mytype 100   # throws "not a valid mytype"

Declaring New Types

intertype.declare() allows to add new type specifications to intertype.specs. It may be called with one to three arguments. The three argument types are:

• type is the name of the new type. It is often customary to call intertype.declare 'mytype', { ... }, but it is also possible to name the type within the spec and forego the first argument, as in intertype.declare { type: 'mytype', ... }.

• spec is an object that describes the type. It is essentially what will end up in intertype.specs, but it will get copied and possibly rewritten in the process, depending on its content and the other arguments. The spec object may have a property type that names the type to be added, and a property tests which, where present, must be an object with one or more (duh) tests. It is customary but not obligatory to name a single test 'main'. In any event, the ordering in which tests are executed is the ordering of the properties of spec.tests (which corresponds to the ordering in which those tests got attached to spec.tests). The spec may also have further attributes, for which see below.

• test is an optional boolean function that accepts one or more arguments (a value x to be tested and any number of additional parameters P where applicable; together these are symbolized as xP) and returns whether its arguments satisfy a certain condition. The test argument, where present, will be registered as the 'main' (and only) test for the new type, spec.tests.main. The rule of thumb is that when one wants to declare a type that can be characterized by a single, concise test, then giving a single anonymous one-liner (typically an arrow function) is OK; conversely, when a complex type (think: structured objects) needs a number of tests, then it will be better to write a suite of named tests (most of them typically one-liners) and pass them in as properties of spec.tests.

The call signatures are:

• intertype.declare spec—In this form, spec must have a type property that names the new type, as well as a tests property.

• intertype.declare type, spec—This form works like the above, except that, if spec.type is set, it must equal the type argument. It is primarily implemented for syntactical reasons (see examples).

• intertype.declare type, test—This form is handy for declaring types without any further details: you just name it, define a test, done. For example, to declare a type for positive numbers: @declare 'positive', ( x ) => ( @isa.number x ) and ( x > 0 ). Also see the next.

• intertype.declare type, spec, test—This form is handy for declaring types with a minimal set of details and a short test. For example, to define a type for NodeJS buffers: @declare 'buffer', { size: 'length', }, ( x ) => Buffer.isBuffer x (here, the size spec defines how InterType's size_of() method should deal with buffers).

Typed Value Casting

XXX TBW XXX

Checks

WIP

• validate.t x, ...—returns true on success, throws error otherwise
• isa.t x, ...—returns true on success, false otherwise
• check.t x, ...—returns any kind of happy value on success, a sad value otherwise

Distinguish between

• isa.t x with single argument: this tests for constant types (including isa.even x which tests against remainder of constant n = 2). isa methods always return a boolean value.

• check.t x, ... with variable number of arguments (which may include previously obtained results for better speed, consistency); this includes check.multiple_of x, 2 which is equivalent to isa.even x but parametrizes n. Checks return arbitrary values; this also holds for failed checks since even a failed check may have collected some potentially expensive data. A check has failed when its return value is sad (i.e. when is_sad check.t x, ... or equivalently not is_happy check.t x, ... is true), and vice versa.

Checks will never throw except when presented with an unknown type or check name.

Checks and types share a common namespace; overwriting or shadowing is not allowed.

sad is the JS symbol intertype.sad; it has the property that it 'is sad', i.e. is_sad intertype.sad returns true.

is_sad x is true for

• sad itself,
• instances of Errors
• all objects that have an attribute x[ sad ] whose value is true.

Conversely, is_sad x is false

• all primitive values except sad itself,
• for all objects x except those where x[ sad ] === true.

One should never use r is sad to test for a bad result, as that will only capture cases where a checker returned the sad symbol; instead, always use is_sad r.

There is an equivalence (invariance) between checks, isa-tests and validations such that it is always possible to express one in terms of the other, e.g.

check_integer     = ( x ) -> return try x if ( validate.integer x ) catch error then error
isa_integer       = ( x ) -> is_happy check_integer x
validate_integer  = ( x ) -> if is_happy ( R = check_integer x ) then return R else throw R

Concatenating Checks

Since checks never throw the programmer must be aware to check for sad results themself. It's advantageous to not use nested if/then/else statements as that would quickly grow to a mess; instead, put related checks into a function on their own and return as soon as any intermediate result is sad, then return the result of the last check.

Another idiom is to use a loop (or wjhile ( true ) { ... }) construct and break as soon as a sad intermediate result is encountered; not to be forgotten is the final break statement that is needed to keep the code from looping indefinetly:

R     = null
loop
  break if ( R = check_fso_exists    path, R ) is sad
  break if ( R = check_is_file       path, R ) is sad
  break if is_sad ( R = check_is_json_file  path, R )
  break
if is_sad R then  warn "fails with", ( rpr R )[ ... 80 ]
else              help "is JSON file; contents:", ( jr R )[ ... 100 ]

Formal Definition of the Type Concept

For the purposes of InterType, a 'type' is reified as (given by, defined by, represented as) a pure, named function t = ( x ) -> ... that accepts exactly one argument x and always returns true or false. Then, the set of all x that are of type t are those where t x returns true, and the set of all x that are not of type t are those where t x returns false; these two sets will always be disjunct (otherwise t cannot be pure, invalidating the premise).

Two trivial functions are the set of all members of all types, any = ( x ) -> true, and the set of values (in the loose sense, but see value and nowait) that have no type at all, none = ( x ) -> false; the former set contains anything representable by the VM at all, while the latter is the empty set (i.e. all values have at least one type, any).

Observe that the above definition implies that any and all JS pure functions of arity one that always return a boolean define a type, even if unintentionally so; for example is_legal_input = ( x ) -> ( x is 42 ) or ( x is 'foo' ) implicitly defines a weird type with the weird name 'is_legal_input' that has exactly two members, an integer number and a three-character string. Less weird and more commonly used are such types that include only a small, enumerable set of values, as in traffic_light_color = ( x ) -> x in [ 'red', 'amber', 'green', ], otherwise known as 'enumerations', or a smallish set defined by pattern matching, as in file_sequence_nr = ( x ) -> ( isa.text x ) and ( x.match /^nr[0-9]{3}$/ )? (which allows nr031 but prohibits nr03x).

Observe that in the last example, it is imperative to first test for x being a text before trying to use the String.prototype.match() method, this to ensure no exception will ever occur. The alternatives are clearly inferior:

• One could try to call x.match() and then catch errors and return false instead; however, this will make arbitrary objects like { match: ( -> true ), } pass the test which is probably not intended.

• It is possible to String::match.call x, pattern, but that will throw for values like null and undefined so still needs to be guarded with try and catch.

As for the x in [ ... ] check, such a safeguard is not needed, but observe that ( new String 'abc' ) in [ 'abc' ] gives false which probably does indeed do what you wanted (namely, exclude those problematic and vexing boxed (wrapped) values) that have no justification to be used, ever.

That a 'type' 'is' a function of a certain kind is indeed a desirable property. First of all, it makes deciding whether a given thing is a type (in almost all cases: trivially) testable. Next, it specifies an unambiguous method how to construct types, and the method of construction is using first principles—unary, boolean pure functions, about the most elementary kind of callables. Not least, it assures us that all functions that are only composed of calls to type definitions and logical operators define a type, too (even if some of those happen to be synonymous to existing types or equivalent to trivial types like any or all); in particular, this means that unions (generalizations) of types according to this definition are unequivocally types according to this definition, too, as are intersections (refinements) of types. And, of course, some functions that go beyond combining function calls by means of and, or, not can shown to be materially types in the sense of this definition. Conversely, we can also be sure that any and all functions that at least for some inputs will call an impure function cannot be said to represent types (unless they try, catch and handle possible exceptions and turn them into a boolean).

As for whether one should encourage or discourage synonymous types—types with multiple names and definitions but identical element sets—the policy is that unwarranted duplication is, of course, to be avoided, but clarity and specificity are desirable. In other words, when you find yourself writing validate.integer x a lot in a single module, chances are that you should really declare a custom type declare mytype = ( x ) -> isa.integer x even if that at the moment is nothing more than replicating an existing definition. If you find yourself writing things like validate.positive_integer x; validate.even x then you should almost certainly define a type that checks for ( isa.positive_integer x ) and ( isa. even x). Also observe that while the body of a type declaration as such are extensional—that is, stating the material tests a given value must pass in order to conform to a given type—the names and the usage of types should tend to be intentional, that is, express fitness for a purpose. Thus, one may want to separately define, say, file_count and line_count: while both are counts (zero or a positive natural number), they count different things and may, in a software system, be subject to different constraints.

immediate and nowait

The type immediate is defined as the complement of promises, that is, the set of all values x for which isa.promise x returns false (so neither native promises nor any 'thenables'—objects where x.then is a function).

The immediate type has been defined as a convenient way to ensure that a given synchronous function call was actually synchronous, i.e. did not return a promise; this may be done as

validate.immediate r = my_sync_function 'foo', 'bar', 'baz'

Observe that immediates do comprise NaN, null, undefined, false and anything else except for promises, so x? is distinct from isa.immediate x.

Equivalently and more succinctly, the validation step can be written with nowait():

nowait r = my_sync_function 'foo', 'bar', 'baz'

nowait x will always either throw a validation error (when x is a promise) or else return x itself, which means that we can write equivalently:

r = nowait my_sync_function 'foo', 'bar', 'baz'

At least in languages with optional parentheses like CoffeeScript, this looks exactly parallel to

r = await my_async_function 'foo', 'bar', 'baz'

hence the name.

To Do

• Allow to pass in target object at instantiation, so e.g. new intertype @ will cause all InterType methods to become available on target as @isa(), @validate and so on.

• Rename export_modules() to export(), allow target object (e.g. module.exports) to be passed in.

• Add types empty, nonempty, ...

• Implement method to iterate over type names, specs.

• Catch errors that originate in type checking clauses

• Trace cause for failure in recursive type checks

• Allow to declare additional casts after type has been declared

• Unify registration of checks and types; rename declare() to declare_type()

• disallow extra arguments to isa(): all typechecks must use exactly one argument (x)

• should undefined be an inherently sad (like errors) or happy (like null) value?

• implement generic checks like equals()

• all checks should be usable with validate, isa

• implement panic()-like function that throws on sad values (keeping exceptions as such, unwrapping saddened values)

• consider whether to return type as intermediate happy value for type checks like if is_happy ( type = check.object x ) then ...

• implement custom error messages for types and/or hints what context should be provided on failure in validations, checks; this in an attempt to cut down on the amount of individual error messages one has to write (ex.: validate.number 42, { name, foo, bar } could quote second argument in error messages to provide contextual values name, foo, bar)

• implement validate.immediate x to check x is anything but a promise; also offer as nowait method (the counterpart to await)

• v4.x.x type declarations should have keys isa (single test or list or object with tests), default (a value that represents initial value of a given type), check (like isa but for checks), sample (generate random values from the type's domain as done in Clojure spec)