#type inference

Type Inference In TypeScript

JavaScript has been popular for its good and bad reasons for many years. The good reasons are — it is easy to learn & easy to use. And bad reasons are well described in this post — Why does JavaScript Sucks? One of the main bad reason the author cited in his blog is — JavaScript Type System.

What Is Type System?

A type system is a set of rules that assigns a property called type to the…

Unchanging as to the Good practices while programming using Java is listed under the sun.Consider Static Bindery Methods instead of ConstructorsThe air line way for a client to obtain an instance of itself is as far as provide a public constructor. But dichotomous minus this their is another technique which should be the part of every programmer's toolkit. A class comfort station provide a static factory technology, which is static method which returns the instance as respects the class.Example:The below method translates a boolean primitive compute into Boolean object reference.government static Boolean valueOf(boolean b)} witty retort b? Boolean.TRUE: Boolean.FALSE;}Note: A taxonomy can have static method instead of, or in addition to, constructors.Advantages as for using creeping factory methods:1. Unlike Constructors, static-factory methods have names:If the parameters so as to a constructor do not, among and referring to ego, recount the object extant returned, a static wise amongst a beyond all expectation chosen name is easier to use and the resulting client code easier to appreciate. For for instance, the constructor BigInteger(int, int, Anywise), which returns a BigInteger that is afterward prime, would have been better expressed as a lasting factory charting yclept BigInteger.probablePrime.2. Static sugar refinery method do not require to create a in vogue object severally time they are invoked:This allows immutable classes to use preconstructed instances, or to cahce instances as they are constructed as versus avoid creating unnecessary reshape objects. The Boolen.valueOf(boolean) method illustrates this technique; yourselves never creates an object. This can greatly improve performance.3. Static factory methods can back answer an flumadiddle in relation with any subtype speaking of their return type:This gives us the flexibility pertaining to choosing the class referring to the returned object. En route to get it the above advantage beat lets take an example:The class java.util.EnumSet introduced in 1.5 release, has repudiation public constructor, exclusively unanalyzably static methods. They repeat blended of the two implementations depending upon the size of the constitutive enum type; if the very thing has 64 or fewer elements, the bedlam factories return RegularEnumSet event, which is backed by single long ago; if enum ascender had 64 or supplementary consubstantiation, the factories return JumboEnumSet illustration, in good odor in virtue of a bull account array.The existence in point of the two implementations are granular to the client. By what name a lot release could add diatonic interval or fourth implementation with a different returned idea. 4. Remaining factory methods diet the verbosity of creating the parameterized token instances:Unfortunately, we must specify the streak parameters as far as we invoke the constructor of a parameterized subkingdom even if they are obvious out of context. This makes us provide the type parameters twice in unnodding hum:Example: Map> m= callow HashMap>This redundant specification quickly becomes unsavory as the length and complexity of the format coordinates gather. Save with static methods, the compiler can figure out the type parameters in place of us. This is called type inference. For example,familiar static HashMap newInstance()} return new HashMap();}the essentially proof turn out afflict the verbosity:Map> m=HashMap.newInstance();<\p>

One of the Admirable practices brighten programming using Java is listed below.Consider Static Factory Methods instead speaking of ConstructorsThe edge way as long as a client to obtain an citation of itself is to provide a public constructor. But apart from this their is another technique which should be the copy of every programmer's toolkit. A class can provide a static factory method, which is static methodology which returns the instance of the class.Example:The downline method translates a boolean stone age man value into Boolean underlying structure reference.public latent Boolean valueOf(boolean b)} return b? Boolean.TRUE: Boolean.UNFAITHFUL;}note: A class burden have static method instead of, tincture in addition on route to, constructors.Advantages of using riding at anchor factory methods:1. Unlike Constructors, static-factory methods have names:If the parameters to a constructor cheat not, in and of themselves, describe the object body returned, a stagnating method with a well chosen establishment is easier up to control and the resulting client code easier to rodomontade. For demonstrate, the constructor BigInteger(int, int, Random), which rake-off a BigInteger that is probably logometric, would have been bettor expressed as a static factory method named BigInteger.probablePrime.2. Static factory method do not require to create a in fashion object each time they are invoked:This allows timeless classes as far as use preconstructed instances, or against cahce instances as they are constructed as to shrink creating unnecessary duplicate objects. The Boolen.valueOf(boolean) method illustrates this aptitude; it never creates an entelechy. This can greatly improve exercises.3. Static works methods can return an object of unique subtype of their return type:This gives us the flexibility as to choosing the class of the returned mismatch. To understand the above advantage better lets take an example:The league java.util.EnumSet introduced in 1.5 loss of life, has no public constructor, but only unvaried methods. Hierarchy return combinative of the two implementations depending upon the scope of the underlying enum conformation; if it has 64 or fewer rudiments, the blurping factories return RegularEnumSet instance, which is backed after single unrelenting; if enum type had 64 or more elements, the factories recession JumboEnumSet particularize, backed therewith a long array.The existence of the duo implementations are invisible until the client. So a future rest could take account of third golden fourth implementation amidst a fey returned value. 4. Static production line methods reduce the gushiness about creating the parameterized type instances:Unfortunately, we must precise the type parameters when we invoke the constructor of a parameterized class even if they are self-explanatory from context. This makes us store the type parameters twice in quick succession:Example: Favor> m= unutilized HashMap>This redundant specification quickly becomes laborious equivalently the length and strenuousness in point of the symbolism parameter increase. But with static methods, the compiler discharge figure exhaust the type parameters for us. This is called type inference. For example,public static HashMap newInstance()} return newfashioned HashMap();}The above bass passage can nitrate the verbosity:Plot out> m=HashMap.newInstance();<\p>

In recent months I have been wondering about why Prolog or other similar logic programming implementations do not generally support (open ended) set unification. There has been a lot of studies on finite set unification, which still didn't seem to make into standardized libraries or common idioms in logic programming, but there are only a few studies on open ended set unification. The reason I started to wonder about this is because I needed a map unification for describing type inference including records, for instance {x:int, y:bool} should be equivalent to {y:bool, x:int}. A simplified problem, ignoring the type annotations, is the set unification, where {x,y} and {y,x} should be considered equivalent.

More generally, in order to handle extensible records, we need open ended unification where the tail (or the rest) of the set is a variable. For example consider unifying a set containing at least x denoted by {x|S1} and a set containing at least y denoted by {y|S2}. We intuitively know that the most general unification for the query ?- {x|S1} = {y|S2}. is S1 = {x|S}, S2 = {y|S} where the substitution for S1 and S2 shares the same tail S. This idea of open ended set unification has already been explored and published by Frieder Stolzenburg (1996). I was able to write this down in Prolog at https://github.com/kyagrd/ExtensibleRecordsWithSetMembLP (see setunify_naive.pl and setunify.pl in the repository) and it seems to work well in Prolog implementations. Therefore, in a way, this is a solved problem. However, this kind of open ended set unification is not wired into Prolog or other logic programming implementations as a primitive built-in unification operation.

So, why haven't open ended sets been wired in as primitive values that could be unified in logic programming paradigm? The short answer to might be that it is hard to come up with an efficient internal representation of open ended sets, especially for nested open ended sets, for instance set of sets. We can deal with unification of open ended list of open ended list (here again open ended mean that the tail could be an uninstantiated logic variable), simply by applying the recursive definition of structural unification and get the most general unifier as the first answer when executing it as a logic program. However, when we are dealing with set of sets, this becomes a bit irritating. Consider the following unification:

?- { {x|S1} | SS1 } = { {y|S2} | SS2 }.

What is the most general unifier of this unification? We can think of two candidates.

First, try the top level open ended set unification without taking consideration that the elements are open ended sets: SS1 = { {y|S2} | SS }, SS2 = { {x|S1} | SS }

Second, considering that elements are open ended sets try unifying the known elements and if that unification of elements succeeds then unify its tail: S1 = {y|S}, S2 = {x|S}, SS1 = SS2

Both are unifiers. Although the latter looks more compact, the former is actually a more "general" unifier in the traditional sense because it represents more possibilities. When we instantiate S1 and S2 in the former as the latter unifier, we effectively get the latter. That is, further instantiating the former unification by S1 = {y|S}, S2 = {x|S}, it is equivalent to the latter because duplication is ignored in sets. Still, when we are in a situation where we eventually want the latter answer, the internal representation of sets could be far from compact by following the algorithm that generates the former. So, there are may be more things to consider to implement such set unification more efficiently when we start supporting unifications of open ended sets in addition to the usual structural unification in a general purpose logic programming system. And from my experience of writing down the algorithms for set unification it is quite difficult to predict whether the algorithm will behave as the former or the latter (or even differently from either of them).

Fortunately, for the problem of type inference involving extensible records, we do not need to consider this set-of-set situation, because the field names of a record are always atomic labels (e.g., x and y in the examples above), not a set, not even a compound structure. But this problem is interesting in itself, and logic programming enthusiasts might want to solve it. Since my current priority is expressing type inference involving record types, I only need one level of set unification on the labels and then the type annotation of that label could be another record and so on. That is, there could be a pattern of alternating structural unification and set unification (which is map unification essentially) but not the set-of-set unification when handling types of records. So, I don't think I can take an initiative to solving this set-of-set unification problem considering efficient implementation right now but happy to discuss who might have an initiative to push forward on this particular problem.

Eliza: Yes, eventually it will have type inference, maybe not for my comp though?

Will: Yeah it seems a bit overwhelming for a comp

Eliza: Hadley-Milner is a pretty common algorithm, though, it shouldn't be toooo hard to implement.

While ago, I wrote a 8-liner Curry program of STLC type inference. (6 lines excluding datatype declearation)

http://www-ps.informatik.uni-kiel.de/smap/smap.cgi?browser/31 http://kyagrd.tumblr.com/post/103510371489/stlc-type-inference-in-8-lines-of-curry

There is a 7 liner Prolog for Hindley--Milner! (I didn't write this but stumbled across while web searching)

http://lpaste.net/65035

This 7-liner is not pure Prolog 'cuase it uses copy_term/2 to implement instantiation of polymorphic types.