A domain-specific language (DSL) is a context-free programming language, created for a specific purpose, which can express tasks from a certain domain. A DSL consists of symbols and operators upon these symbols. A DSL is described as a context-free grammar – i.e., as a set of rules, where each symbol on the left-hand side is bound to a set of possible operators on the right-hand side that represent this symbol. Every operator of a DSL is pure – it does not have side effects.

A program in a DSL transforms input data into output data. A program is represented as an abstract syntax tree (AST) of the DSL operators. Each node in this tree (similarly, each DSL operator) has some invocation semantics. More formally, each AST node has a method Invoke. It takes as input a state $\mathbf{\sigma}$ and returns some output. A state is a mapping from DSL variables to their values. Initially, the topmost AST node invoked on a state $\sigma = \left\{ v \mapsto i \right\}$, where $v$ is the DSL variable that represents program input, and $i$ is the value of the input data on which the program is invoked. AST nodes can modify this state (add new variable bindings in scope) and pass it down to children nodes in recursive invocations of Invoke.


The Microsoft.ProgramSynthesis package unites the core pieces of the meta-synthesizer. Here are the included assemblies:

  • Microsoft.ProgramSynthesis. includes the core functionality for manipulating ASTs, validating DSLs, maintain a DSL and a set of its operators, and core infrastructure for the synthesis algorithms.
  • dslc.exe is a DSL compiler. This is the executable that takes a language definition, parses it, validates it, potentially emits some diagnostics (warnings and/or errors), and serializes the parsed language representation into a portable file.
  • Microsoft.ProgramSynthesis.Learning is a core library of synthesis algorithms, standard operators and strategies.
  • Microsoft.ProgramSynthesis.Utils is a collection of utility methods that are used in different parts of the project.

A typical workflow of a DSL designer consists of the following steps:

  1. Define a DSL in a separate file (usually has a *.grammar extension).
  2. Define the semantics and all the supporting infrastructure for the DSL in a separate .NET assembly (see Section 4 below).
  3. Use one of the two options below to compile the DSL definition into a Grammar object:
    • At compile-time, invoke the DSL compiler dslc.exe manually on a *.grammar file with a DSL definition. Deploy the resulting serialized *.grammar.xml file with yo ur application. At run-time, deserialize it using Grammar.Deserialize method.
    • Deploy the dslc.exe and the *.grammar file with your application. At run-time, compile your DSL definition in memory using DSLCompiler.Compile method.

Language definition

The main class Grammar represents a context-free grammar as a set of DSL rules and a list of references to operators’ semantics and/or custom operator learners, implemented in C# in a separate assembly. The method DSLCompiler.Compile loads a grammar definition from a string. Multiple examples of its usage can be found in our sample repository.

Here’s a typical language definition:

using TestSemantics;
using semantics TestSemantics.FlashFill;
using learners TestSemantics.FlashFill.Learners;

language FlashFill;

@start string f := ConstStr(s) | let x: string = v in SubStr(x, P, P);
int P := CPos(x, k) | Pos(x, r, r, k);
int k;
@values[StringGen] string s;
Regex r;
@input string v;

NOTE: The operators ConstStr, SubStr, CPos and Pos are defined in Black-box operators below.

First, we reference any namespaces for external typename lookup. Then, we specify the static classes for semantics and learners. There may be several using, using semantics, and using learners instructions. In this example, TestSemantics.FlashFill is a static class defined in the assembly “TestSemantics.dll” (which we pass as a reference in the DSLCompiler.Compile invocation) as is TestSemantics.FlashFill.Learners.

Next, we specify the language name and its rules in a form of a context-free grammar (BNF). Each nonterminal rule has a symbol on the left-hand side and a list of operators on the right-hand side, separated by “|”. Each terminal rule has only the left-hand side. Each symbol has its type, specified on the LHS. The supported types are: any of the standard C# types, including classes from System.Collection.Generic, System.Text.RegularExpressions, etc., or your own custom types, defined in the same semantics assembly. In the latter case, the framework searches for the type name in the namespaces referenced in using instructions.

Exactly one nonterminal rule should be annotated as “@start” – this is a start symbol of the grammar (i.e., the topmost node of every AST). Exactly one terminal rule should be annotated as “@input” – this is the input data to the program.


Terminal rules specify the leaf symbols that will be replaced with literal constants or variables in the AST. For example:

  • A terminal rule int k; specifies a symbol $k$ that represents a literal integer constant.
  • A terminal rule @input string v; specifies a variable symbol $v$ that will be replaced with program input data at runtime.

A user can specify the list of possible values that a literal symbol can be set to. This is done with a @values[$G$] annotation, where $G$ is a value generator – a reference to a user-defined static field, property, or method. $G$ should evaluate to IEnumerable (thus, it can be any standard .NET collection, such as an array or a list, or a user-defined collection type). The framework searches for the definition of $G$ in the learner classes, specified by a using learners instruction. For our example grammar, any of the following definition can serve as a value generator for $s$:

public static class FlashFill
    public static class Learners
        public static string[] StringGen = {"", "123", "Gates, B.", "ABRACADABRA"};
        public static string[] StringGen
            get { return new[] {"", "123", "Gates, B.", "ABRACADABRA"}; }
        public static string[] StringGen()
            return new[] { "", "123", "Gates, B.", "ABRACADABRA" };

If no value generator is provided, the framework can either pick a standard generator for some common types (such as byte), or assume that the literal can be set to any value. This can impact the performance of some synthesis strategies or even make them inapplicable.

Black-box operators

A black-box operator is an operator that does not refer to any standard concepts, and its invocation semantics do not modify the state $\sigma$ that has been passed to it. In the example above ConstStr, SubStr, CPos, Pos are black-box operators. Note that SubStr is a black-box operator, even though the Let concept surrounding it isn’t. SubStr’s semantics don’t modify $\sigma$, they just pass it on to parameter symbols. Let’s semantics change the state $\sigma$ by extending it with a new variable binding $x \mapsto v$.

The semantics of every black-box operator should be defined in the semantics assembly with the same name as a public static method:

public static class Semantics
    public static int CPos(string s, int k)
        return k < 0 ? s.Length + 1 + k : k;

    public static int? Pos(string s, Regex leftRegex, Regex rightRegex,
                           int occurrence)
        var boundaryRegex = new Regex(string.Format("(?<={0}){1}", leftRegex, rightRegex));
        var matches = boundaryRegex.Matches(s);
        int index = occurrence > 0 ? occurrence - 1 : matches.Count + occurrence;
        if (index < 0 || index >= matches.Count)
            return null;
        return matches[index].Index;

    public static string ConstStr(string s)
        return s;

    public static string SubStr(string s, int left, int right)
        if (left < 0 || right > s.Length || right < left)
            return null;
        return s.Substring(left, right - left);

    public static string Concat(string s, string t)
        return s + t;

It is important to note that the learning process requires that semantic functions are total. If the function is inapplicable for the current choice of arguments or if for any other reason it would throw an exception, it must return null instead1. In such a case, if the return type of a function is a .NET value type, it should be made nullable.

This choice was made to enable efficient program synthesis. During learning, PROSE may repeatedly invoke partial programs on various inputs for verification purposes. If an input is invalid and a program handles it by throwing an exception, it slows down the learning by two orders of magnitude.

Nulls are automatically propagated upward like exceptions: if an argument to a semantic function is null, its result is automatically presumed to be null as well. You can override this behavior by annotating your semantics function with a [LazySemantics] attribute. In that case, your function will receive any null arguments as usual, and must handle it on its own.

Every grammar operator should be pure, and its formal signature should be $\sigma \rightarrow T$ (where $T$ is the type of the corresponding LHS symbol). The actual signature of the semantics function that implements a black-box operator is $\left( T_{1},T_{2},\ldots,T_{k} \right) \rightarrow T$, where $T_{j}$ is the type of $j^{\mathrm{\text{th}}}$ parameter symbol, and $T$ is the return type of the LHS symbol. Consequently, if one needs to invoke their operators on anything other than the program input data, they have to introduce additional variables using Let construct and/or lambda functions.

Let construct

Let construct is a standard concept with the following syntax:

$$ \text{let } x_{1}\colon \tau_1 = v_{1},\,\ldots,\,x_{k}\colon \tau_k = v_{k}\text{ in } RHS $$

where $RHS$ is any standard rule RHS (a black-box operator, a grammar symbol, etc.). The RHS and any of the symbols it (indirectly) references can make use of the variables $x_1,\ldots,x_k$ Grammar symbols $v_1,\ldots,v_k$ are parameters of the rule; at runtime, each variable $x_j$ is bound to some value of the corresponding symbol $v_j$.


The running grammar definition FlashFill contains the following rule:

@start string f := let x: string = v in SubStr(x, P, P)

The formal signature of this rule has three free parameters: $v,\ P,\ P.$ At runtime, the Let construct, when given a state $\sigma$, executes the following operations:

  • $\vartheta := \llbracket v \rrbracket \sigma$
    (Execute the parameter symbol $v$ on the state and save the value as $\vartheta$)
  • $\sigma^{'} := \sigma \cup \left\{ x \mapsto \vartheta \right\}$
    (Add a new variable $x$ to the state, bind it to the value $\vartheta$)
  • $\mathrm{\text{return }} \llbracket SubStr(x,P,P) \rrbracket \sigma’$
    (Invoke the RHS – the black-box operator SubStr – on the new state)

The RHS indirectly references the variable $x$ further in the grammar through the symbol $P$. Namely, the grammar further contains the following rule:

int P := Pos(x, r, r, k)

Pos is a simple black-box operator, just like SubStr. When given a state $\sigma’$, it passes it on to its parameter symbols $x,r,r,k$. The latter three symbols are terminals; they will be represented as literal constants of the corresponding types in any final program (AST). The first symbol is a variable $x$; when executed on a state $\sigma’$, it will just extract the binding $x \mapsto \vartheta$ from it, and return $\vartheta$.

Standard concepts

There exists a range of concepts that are common for many DSLs and implement standard functionality. In particular, many list/set processing concepts (Map, Filter, TakeWhile, etc.) encode various forms of loops that arise in many DSLs for different purposes. The synthesizer treats many such concepts as first-class citizens and is aware of many efficient strategies for synthesizing such concepts from example specifications. Consequently, we encourage framework users to use standard concepts for any loop form that arises in their DSLs, instead of encoding it as a black-box operator. In our experience, most loop forms commonly used in DSLs can be expressed as combinations of standard concepts.

The current list of supported concepts can be found in the Microsoft.ProgramSynthesis.Rules.Concepts namespace.

Most standard loop concepts express some form of list/set processing with lambda functions. We explain their usage here on a simple Filter example.

A $Filter(p,\ s)$ concept is an operator that takes as input a predicate symbol $p$ and a set symbol $s$. It evaluates $s$ on a state $\sigma$ to some set $S$ of objects. It also evaluates $p$ on a state $\sigma$ to some lambda function $L = \lambda x:e$. Finally, it filters the set $S$ using $L$ as a predicate, and returns the result. Essentially, $Filter$ is equivalent to Select in LINQ.

Consider the following grammar for various filters of an array of strings:

using TestSemantics;
using semantics TestSemantics.Flare;
using learners TestSemantics.Flare.Learners;

language Flare;

@input string[] v;
Regex r;
StringFilter f := Match(r) | FTrue();
@start string[] P := Selected(f, v) = Filter(f, v);

The input string array $v$ is filtered with a filter $f$. A filter $f$ can filter elements either according to a regular expression $r$, or trivially (by returning true).

The main rule of the grammar is string[] P := Selected(f, v). Here Selected(f, v) is a formal operator signature: this is how it would look like if it was implemented as a black-box operator. However, the actual implementation of Selected(f, v) refers to the standard Filter concept instead of a black-box semantics implementation. The arguments of the Filter concept are in this case the parameters of a Selected operator – the symbols $f$ and $v$. At runtime, the framework interprets a Selected AST node by executing the standard Filter semantics with the corresponding arguments.

For a valid execution, the runtime types of the symbols $f$ and $v$ should satisfy the following contract:

  1. The type of $v$ should implement IEnumerable. It can be a standard .NET collection (array, List<T>, etc.), or a user-defined custom type that implements IEnumerable.

  2. The type of $f$ should have functional semantics. In other words, it should behave like a lambda function, because at runtime it will be “invoked” with array elements as arguments. Moreover, for the Filter concept specifically, the return type of this “function” should be assignable to bool.

Lambda functions

One can capture functional semantics in an explicit lambda function in the grammar:

using semantics TestSemantics.Flare;
using learners TestSemantics.Flare.Learners;

language Flare;

@input string[] v;
Regex r;
bool f := Match(x, r) | True();
@start string[] P := Selected(f, v) = Filter(\x: string => f, v);

The first argument of the Filter concept is a lambda function $\lambda x:f$. In our syntax, it is represented as \x: string => f2. Here $x$ is a freshly bound variable (lambda function parameter), and $f$ is a grammar symbol that represents function body. The runtime type of $x$ should be specified explicitly after a colon.

Just as with Let constructs, the lambda function body symbol and all its indirect descendants can now reference the variable symbol $x$. At runtime, it will be successively bound to every element of the input string array. Since this binding introduces a new variable in the state $\sigma$, a lambda definition cannot be expressed as a black-box rule. Instead, it is a special rule kind with first-class treatment in the framework (again, just as Let).

The corresponding semantics implementation is now much simpler:

public static class Flare
    public static bool Match(string x, Regex r)
        return r.IsMatch(x);

    public static bool True()
        return true;

Note that a lambda function body is a free symbol on the RHS of the “=” sign. In other words, the set of free symbols on the RHS includes the direct parameters of the concept (in the example above, $v$) and the lambda function bodies (in the example above, $f$). To make the concept rule well-defined, this set should be exactly the same as the set of free symbols on the LHS of the “=” sign (i.e. the set of formal parameters of the rule). However, they should only be equivalent as sets, the order of parameters does not matter. In case of multiple usages of the same symbol among parameters, the correspondence between the LHS and the RHS is resolved in a left-to-right order.


Consider the following concept rule:

int S := F(v, P, P) = G(\x: string => P, v, P);

Here $G$ is some standard concept, and $S,\ v,P$ are grammar symbols. The correspondence between formal parameters on the LHS and free symbols on the RHS is resolved as follows:

  • The first parameter $v$ on the LHS corresponds to the second argument $v$ on the RHS.
  • The second parameter $P$ on the LHS corresponds to the body of the first argument $\lambda x:P$ on the RHS.
  • The third parameter $P$ on the LHS corresponds to the third argument $P$ on the RHS.

Two usages of the same symbol $P$ among parameters were resolved in a left-to-right order.

Language usage

Definition and usage of custom DSLs starts with the following steps:

  1. Define a string representation of your DSL grammar in our syntax.
  2. Implement your black-box operator semantics, custom types, and value generators in a separate assembly. Make sure that it is accessible at a path specified in the grammar string3.
  3. Load the grammar into a Grammar object:
    • Option 1: programmatically using DSLCompiler.Compile.

The example below shows how to compile the grammar programmatically (option “a”):

 const string TestFlareGrammar = @"
         using TestSemantics;
         using semantics TestSemantics.Flare;
         language Flare;
         @input string[] v;
         Regex r;
         StringFilter f := Match(r) | FTrue();
         @start string[] P := Selected(f, v) = Filter(f, v);";
var grammar = DSLCompiler.Compile(new CompilerOptions()
        InputGrammarText = TestFlareGrammar,
        References = CompilerReference.FromAssemblyFiles(typeof(Program).GetTypeInfo().Assembly)

Partial programs

In many applications there is need to manipulate partial programs – ASTs where some tree nodes are replaced with holes. A hole is a special type of an AST node: it’s an unfilled placeholder for some instantiation of a corresponding grammar symbol. In the AST node class hierarchy it is represented as Microsoft.ProgramSynthesis.AST.Hole.

To get a list of descendant holes in a program $p$, call p.Holes. Note that in current architecture AST nodes do not maintain any references to their parents. Consequently, to enable practical usages, the Holes property returns not the Hole nodes themselves, but their positions instead. A position is represented as a tuple $(P,\ k,\ H)$, where $P$ is a parent AST node of a hole, $k$ is the hole’s index in the list of $P$’s children, and $H$ is the hole itself.

A string representation of a hole of symbol $S$ is “?S”. The framework supports parsing AST strings that contain holes.



Program synthesis in the PROSE SDK is defined as a process of generating a set of programs $\tilde{P}$ that start with a root symbol $P$, given a specification $\varphi$. A specification is a way of defining the desired behavior of every program in $\tilde{P}$. Different synthesis applications have different kinds of specifications to specify the desired program behavior. Some of the examples from prior work include:

  • In FlashFill, $\varphi$ is an example specification: for every given input state $\sigma$, the desired program output should be equal to the given string $\varphi\lbrack\sigma\rbrack$.

  • In Extraction.Text, $\varphi$ is a subset specification. It assumes that the runtime type of a root symbol $P$ is some sequence IEnumerable<T>. The specification $\varphi$ expresses the following property: for every given input state $\varphi$, the desired program output should be a superset of the given subset $\varphi\lbrack\sigma\rbrack$. In terms of Extraction.Text, $\varphi\lbrack\sigma\rbrack$ are the substrings highlighted by the user, and the program output includes them and the rest of the substrings that should be highlighted.

The supported specification kinds are located in the namespace Microsoft.ProgramSynthesis.Specifications. All of them inherit the base abstract class Specification. This class has a single abstract method Valid$\colon$ (State, object) $\to$ bool, which returns true if and only if the program output on the given input state satisfies the constraint that the specification expresses.

Spec is the main abstract base class for all inductive specifications, i.e. those that constraint the program behavior on a set of provided input states $\varphi$.ProvidedInputs. Some of the main inductive specification kinds are described below:

  • InductiveConstraint specifies an arbitrary constraint Constraint$\colon$ (State, object) $\to$ bool on the set of provided input states.
  • SubsequenceSpec specifies a subsequence of the desired output sequence for each provided input state in the dictionary Examples$\colon$ State $\mapsto$ IEnumerable<object>.
    • PrefixSpec is a subclass of SubsequenceSpec where the desired subsequence of a program output is also required to be its prefix.
  • FunctionalOutputSpec describes the behavior of the program whose output type is a lambda function or a functional symbol. For each input state, it specifies a set of input/output pairs. These pairs are the behavior examples for the function that is the output of a desired program on a given state.
  • DisjunctiveExamplesSpec specifies a set of possible desired outputs for each provided input state. On a given state $\sigma$, the program output is allowed to equal any of the given possible outputs $\varphi$.DisjunctiveExamples$\lbrack\sigma\rbrack$.
    • ExampleSpec is a subclass of DisjunctiveExamplesSpec where the size of $\varphi$.DisjunctiveExamples$\lbrack\sigma\rbrack$ is constrained to 1. In other words, this is the simplest specification that specifies the single desired program output for each provided input state.


The main point of program synthesis in the framework is the SynthesisEngine class. Its function is to execute different synthesis strategies for synthesizing parts of the resulting program. The synthesis process starts with an engine.LearnGrammar($\varphi$) call to synthesize a program that starts with a start symbol of the grammar, or, more generally, engine.LearnSymbol($P$, $\varphi$) call to synthesize a program that starts with a given grammar symbol $P$.

Given a learning task $\left\langle P,\varphi \right\rangle$ to synthesize a program that starts with a symbol $P$ and satisfies the specification $\varphi$, the engine can assign this task to any of the available synthesis strategies. A synthesis strategy represents a specific algorithm that can synthesize a program set $\tilde{P}$ for a particular kind of a learning task $\left\langle P,\varphi\right\rangle$. In other words, a synthesis strategy is parameterized by its supported specification type, takes a learning task $\left\langle P,\varphi\right\rangle$ where $\varphi$ should be an instance of this specification type, and learns a set of programs $\tilde{P}$ that are consistent with $\varphi$.

A synthesis strategy is represented as a class inheriting Microsoft.ProgramSynthesis.Learning.SynthesisStrategy<TSpec>, which specifies the following contract:

public abstract class SynthesisStrategy<TSpec> : ISynthesisStrategy
        where TSpec : Spec 
    void Initialize(SynthesisEngine engine);
    abstract Optional<ProgramSet> Learn(SynthesisEngine engine, LearningTask task, CancellationToken cancel);
    bool CanCall(Spec spec);

Here TSpec is a supported specification type, Learn is the main learning method, and CanCall is the function that determines whether this synthesis strategy supports learning for a given specification spec (in the default implementation, the result is true if and only if spec is an instance of TSpec).

Version space algebra

The return type of Learn in SynthesisStrategy is Microsoft.ProgramSynthesis.VersionSpace.ProgramSet. This is an abstract base class for our representation for the sets of programs. A version space, defined by Mitchell and Lau , is a succinct representation of a program set (hypothesis space in machine learning community), consistent with a specification. A version space can be defined explicitly (as a set of programs), or composed from smaller version spaces using standard set operators. The latter property is a key to succinctness of version spaces: representing exponential sets of programs using composition operators requires only polynomial space. Such a structure defines an algebra over primitive version spaces, hence the name.

An abstract ProgramSet class defines the following contract:

public abstract class ProgramSet : ILanguage
    protected ProgramSet(Symbol symbol)
        Symbol = symbol;

    public Symbol Symbol { get; private set; }

    public abstract IEnumerable<ProgramNode> RealizedPrograms { get; }

    public abstract ulong Size { get; }

    public virtual bool IsEmpty
        get { return RealizedPrograms.IsEmpty(); }

    public abstract ProgramSet Intersect(ProgramSet other);

    public Dictionary<object, ProgramSet> ClusterOnInput(State input);

    public Dictionary<object[], ProgramSet> ClusterOnInputs(IEnumerable<State> input);

    public abstract XElement ToXML();

The property RealizedPrograms calculates the set of programs stored in the version space.

Direct version space

Direct version space is a primitive version space that represents a set of programs explicitly, by storing a reference to IEnumerable<ProgramSet>. Since IEnumerable<T> in .NET is lazy, storing it in a version space does not by itself enumerate it into an explicit set. This allows you to implement a synthesis strategy as an iterator in C# (with yield return statements), which calculates the required number of consistent programs on the fly, as needed. Moreover, such an iterator can even be theoretically infinite, as long as the end-user requests only a finite number of consistent programs.

Union version space

A union of $k$ version spaces is a version space that contains those and only those programs that belong to at least one of the given $k$ spaces. Such a version space naturally arises when we want to learn a set of programs that start with a certain grammar symbol by learning each of its possible RHS rules automatically. For example:

$$ P\ : = F\left( A,\ B \right) \mid G\left( C, D \right) $$

Here $P,A,B,C,D$ are grammar symbols, and $F,G$ are black-box operators. Given a learning task $\left\langle P,\varphi \right\rangle$, one way to resolve it is to independently learn a set of programs $\tilde{F}$ of form $F(?A,\ \ ?B)$, and a set of programs $\tilde{G}$ of form $G(?C,\ \ ?D)$. If all the programs in $\tilde{F}$ and $\tilde{G}$ are consistent with $\varphi$, then $\tilde{F} \cup \tilde{G}$ is a valid answer to the outer learning task $\left\langle P,\varphi \right\rangle$.

Join version space

A join version space is defined for a single operator $P := F\left( E_{1},\ldots,E_{k} \right)$. It represents a set of programs $\tilde{P}$ formed by a Cartesian product of parameter version spaces $\tilde{E_{1}},\ldots,\tilde{E_{k}\ }$. In general, not every combination of the parameter programs from this Cartesian product is a valid sequence of parameters for the operator $F$, thus join version space depends on the operator logic to filter out the invalid combinations.

The table below outlines the APIs for building version spaces in the Microsoft Program Synthesis using Examples framework:

Version space Given Builder code
Empty Symbol s ProgramSet.Empty(s)
Explicit list of programs Symbol s
ProgramNode p1, …, pk
ProgramSet.List(s, p1, …, pk)
Lazy stream of programs Symbol s
IEnumerable<ProgramNode> stream
new DirectProgramSet(s, stream)
Union of version spaces for the rules Symbol s
ProgramSet v1, …, vk
new UnionProgramSet(s, v1, …, vk)
Join of version spaces for the parameters Symbol s
GrammarRule r // r.Head == s
ProgramSet v1, …, vk
new JoinProgramSet(r, v1, …, vk)

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In the meantime, check out our sample code to learn the API.
  1. In other words, null is used as a special value $\bot$ that is typically found in a formal definition of a language. 

  2. Following Haskell syntax, we start our lambda functions with the “\” character, which is supposed to approximately represent the letter $\lambda$. 

  3. The common recipe that we use in our development is to reference the semantics DLL from the main project in a Visual Studio solution. This way, the semantics DLL is automatically copied to the target subdirectory on each build next to the main executable, and you can refer to it in the grammar string by simply using its filename. Alternatively, you can specify additional library paths as extra parameters to the Compile method.