Getting Started with Terra

Zach DeVito (zdevito at cs dot stanford dot edu)

terralang.org

Terra is a new low-level system programming language that is designed to interoperate seamlessly with the Lua programming language. It is also backwards compatible with (and embeddable in) existing C code. Like C, Terra is a monomorphic, statically-typed, compiled language with manual memory management. But unlike C, it is designed to make interaction with Lua easy. Terra code shares Lua's syntax and control-flow constructs. It is easy to call Lua functions from Terra (or Terra functions from Lua).

Furthermore, you can use Lua to meta-program Terra code. The Lua meta-program handles details like conditional compilation, namespaces, and templating in Terra code that are normally special constructs in low-level languages. This coupling additionally enables more powerful features like function specialization, lisp-style macros, and manually controlled JIT compilation. Since Terra's compiler is also available at runtime, it makes it easy for libraries or embedded languages to generate low-level code dynamically.

This guide serves as an introduction for programming in Terra. A general understanding of the Lua language is very helpful, but not strictly required.

Installing Terra

Terra currently runs Mac OS X and Linux. Terra uses LLVM 3.2, Clang 3.2 (the C/C++ frontend for LLVM), and LuaJIT 2.0 -- a tracing-JIT for Lua code. Terra will download and compile LuaJIT for you, but you will need to install Clang and LLVM. The easiest way to do this is to the download the Clang Binaries (which also include LLVM binaries) from the LLVM download page.

Unzip the tar-ball and then copy it into /usr/local (or somewhere else that is in your $PATH):

$ tar -xf clang+llvm-3.2-x86_64-apple-darwin11.tar.gz
$ cp -r clang+llvm-3.2-x86_64-apple-darwin11/ /usr/local

Clang should now report being version 3.2:

$ clang --version
clang version 3.2 (tags/RELEASE_32/final)
Target: x86_64-apple-darwin12.2.1
Thread model: posix

Run make in the terra directory to download LuaJIT and build Terra:

$ make

Running Terra

Similar to the design of Lua, Terra can be used as a standalone executable/read-eval-print-loop (REPL) and also as a library embedded in a C program. This design makes it easy to integrate with existing projects.

To run the REPL:

$ ./terra

Terra -- A low-level counterpart to Lua

Stanford University
[email protected]

> 

Terra's REPL behaves similar to Lua's REPL. If you are familiar with other languages like Python, the one major difference is that expressions must be prefixed with return or = if you want to get their value:

> 3        --ERROR! it is expecting a statement
stdin:1: unexpected symbol near 3
> return 3 -- OK!
3
> = 3      -- syntax sugar in the REPL for 'return 3'
3

You can also run it on already written files:

$ ./terra tests/hello.t
hello, world

Terra can also be used as a library from C by linking against libterra.a. The interface is very similar that of the Lua interpreter. A simple example initializes Terra and then runs code from the file specified in each argument:

#include <stdio.h>
#include "terra.h"

int main(int argc, char ** argv) {
    lua_State * L = luaL_newstate(); //create a plain lua state
    luaL_openlibs(L);                //initialize its libraries
    //initialize the terra state in lua
    terra_init(L);
    for(int i = 1; i < argc; i++)
    	//run the terra code in each file
        if(terra_dofile(L,argv[i]))  
            exit(1);
    return 0;
}

In addition to these modes, Terra code can be compiled to .o files which can be linked into an executable, or even compiled to an executable directly.

A bunch of example scripts can be found in the tests/ directory. The run script in the directory will run all of these languages tests to ensure that Terra is built correctly.

Hello, World

Hello world is simple:

print("hello, world")

This program is actually a completely valid Lua program as well. The top-level declarations in a Terra source code file are always run as normal Lua code! This top-level Lua layer handles the details like conditional compilation, namespaces, and templating of terra code. We'll see later that it additionally allows for more powerful meta-programming features such as function specialization, and multi-stage programming.

To actually begin writing Terra code, we introduce a Terra function with the keyword terra:

terra addone(a : int)
    return a + 1
end

print(addone(2)) --this outputs: 3

Unlike Lua, arguments to Terra functions are explicitly typed. Terra uses a simple static type propagation to infer the return type of the addone function. You can also explicitly specify it:

terra addone(a : int) : int
	return a + 1
end

The last line of the example invokes the Terra function from the top-level context. This is an example of the interaction between Terra and Lua. Terra code is JIT compiled to machine code when it is first needed. In this example, this occurs when addone is called. In general, functions are needed when then are called, or when they are referred to by other functions that are being compiled.

More information on the interface between Terra and Lua can be found in Lua-Terra interaction.

We can also print "hello, world" directly from Terra code like so:

local C = terralib.includec("stdio.h")

terra main()
    C.printf("hello, world\n")
end

main()

The function terralib.includec is a Lua function that invokes Terra's backward compatibility layer to import C code in stdio.h into the Lua table C. Terra functions can then directly call the C functions. Since both clang (our C frontend) and Terra target the LLVM intermediate representation, there is no additional overhead in calling a C function. Terra can even inline across these calls if the source of the C function is available!

The local keyword is a Lua construct. It introduces a locally scoped Lua variable named C. If omitted it would create a globally scoped variable.

You can also compile code into a .o, or compile a stand-alone native executable. We can instruct the Terra compiler to save an object file or executable:

-- save a .o file you can link to normal C code:
terralib.saveobj("hello.o",{ main = main })

-- save a native executable
terralib.saveobj("hello", { main = main }) 

The second argument is a table of functions to save in the object file and may include more than one function. The implementation of saveobj is still very primitive. For instance, it won't correctly save Terra functions that invoke Lua functions. This interface will become more robust over time.

Variables and Assignments

Variables in Terra code are introduced with the var keyword:

terra myfn()
    var a : int = 3
    var b : double
end

Unlike Lua, all Terra variables must be declared. Initializers are optional. b's value above is undefined until it is assigned. If an initializer is specified, then Terra can infer the variables type automatically:

terra myfn()
    var a = 3.0 --a will have type double
end

You can have multiple declarations on one line:

terra myfn()
    var a : int, b : double = 3, 4.5
    var c : double, d       = 3, 4.5
end

Lua and Terra are both whitespace invariant. However, there is no need for semicolons between statements. The above statement is equivalent to:

terra myfn()
    var a : int, b : double = 3, 4.5 var c : double, d = 3, 4.5
end

If you want to put a semicolon in for clarity you can:

terra myfn()
    var a : int, b : double = 3, 4.5; var c : double, d = 3, 4.5
end

Assignments have a similar form:

terra myfn()
    var a,b = 3.0, 4.5
    a,b = b,a 
    -- a has value 4.5, b has value 3.0 
end

As in Lua, the right-hand side is executed before the assignments are performed, so the above example will swap the values of the two variables.

Variables can be declared outside terra functions as well:

a = global(double,3.0)
terra myfn()
    return a
end

This makes a a global variable that is visible to multiple Terra functions. The global function is part of Terra's Lua-based API. It initializes a to the Lua value 3.0.

Variables in Terra are always lexically scoped. The statement do <stmts> end introduces a new level of scoping (for the remainder of this guide, the enclosing terra declaration will be omitted when it is clear we are talking about Terra code):

var a = 3.0
do
    var a = 4.0
end
-- a has value 3.0 now

Control Flow

Terra's control flow is almost identical to Lua except for the behavior of for loops.

If Statements

if a or b and not c then
    C.printf("then\n")
elseif c then
    C.printf("elseif\n")
else
    C.printf("else\n")
end

Loops

var a = 0
while a < 10 do
    C.printf("loop\n")
    a = a + 1
end

repeat
    a = a - 1
    C.printf("loop2\n")
until a == 0

while a < 10 do
    if a == 8 then
        break
    end
    a = a + 1
end

Terra also includes for loop. This example counts from 0 up to but not including 10:

for i = 0,10 do
    C.printf("%d\n",i)
end

This is different from Lua's behavior (which is inclusive of 10) since Terra uses 0-based indexing and pointer arithmetic in contrast with Lua's 1-based indexing. Ideally, Lua and Terra would use the same indexing rules. However, Terra code needs to frequently do pointer arithmetic and interface with C code both of which are cumbersome with 1-based indexing. Alternatively, patching Lua to make it 0-based would make the flavor of Lua bundled with Terra incompatible with existing Lua code.

Lua also has a for loop that operates using iterators. This is not yet implemented (NYI) in Terra, but a version will be added eventually.

The loop may also specify an option step parameter:

for i = 0,10,2 do
    c.printf("%d\n",i) --0, 2, 4, ...
end

Gotos

Terra includes goto statements. Use them wisely. They are included since they can be useful when generating code for embedded languages.

::loop::
C.printf("y\n")
goto loop

Functions

We have already seen some simple function definitions. In addition to taking multiple parameters, functions in Terra (and Lua) can return multiple values:

terra sort2(a : int, b : int) : {int,int} --the return type is optional
    if a < b then   
        return a, b
    else
        return b, a
    end
end

terra doit()
    var a,b = sort2(4,3)
    --now a == 3, b == 4
end
doit()

As mentioned previously, compilation occurs when functions are first needed. In this example, when doit() is called, both doit() and sort2 are compiled because doit refers to sort2.

Mutual Recursion

Symbols such as variables and types are resolved when a function is defined. The following example results in an error because isodd is not declared when iseven is defined:

terra iseven(n : uint32)
    if n == 0 then
        return true
    else
    	-- ERROR! isodd has not been defined
        return isodd(n - 1) 
    end
end
 
terra isodd(n : uint32)
    if n == 0 then
        return false
    else
        return iseven(n - 1)
    end
end

You solve this by connecting the definitions with an and. This causes both isodd and iseven to be defined at the same time:

terra iseven(n : uint32)
    if n == 0 then
        return true
    else
    	-- OK! isodd defined at the same time.
        return isodd(n - 1) 
    end
end 
and terra isodd(n : uint32)
    if n == 0 then
        return false
    else
        return iseven(n - 1)
    end
end

Alternatively, you can declare a function before defining it:

terra isodd
terra iseven(n : uint32)
    ...
end 
terra isodd(n : uint32)
   ...
end

Note that unlike C++ it is not necessary to give the type of isodd in the declaration -- though symbols like isodd are resolved eagerly, we only perform type-checking when a function is compiled.

Like Lua function definitions, Terra function defintions can insert directly into Lua tables.

local mytable = {}
terra mytable.myfunction()
C.printf("myfunction in mytable\n")
end

Terra Functions Are Lua Objects

So far, we have been treating terra functions as special constructs in the top-level Lua code. In reality, Terra functions are actually just Lua values. In fact, the code:

terra foo()
end

Is just syntax sugar for*:

foo = terra()
    --this is an anonymous terra function
end

The symbol foo is just a Lua variable whose value is a Terra function. Lua is Terra's meta-language, and you can use it to perform reflection on Terra functions. For instance, you can ask to see the disassembly for the function:

terra add1(a : double)
    return a + a
end

--this is Lua code:
> add1:disas()
definition 	{double}->{double}

define double @add111(double) {
entry:
  %1 = fadd double %0, %0
  ret double %1
}

assembly for function at address 0xa2ef030
0:		vaddsd	XMM0, XMM0, XMM0
4:		ret

You can also force a function to be compiled:

add1:compile()

Or look at a textual representation of the type-check code

> add1:printpretty()
add1 = terra(a : double) : {double}
	return a + a
end

* The actual syntax sugar is slightly more complicated to support function declarations. See the API reference for the full behavior.

Symbol Resolution

When the Terra compiler looks up a symbol like add1 it first looks in the local environment of the terra function. If it doesn't find the symbol, then it simply continues the search in the enclosing (Lua) environment. If the compiler resolves the symbol to a Lua value, then it converts it to a Terra value where possible. Let's look at a few examples:

local N = 4
terra powN(a : double)
    var r = 1
    for i = 0, N do
        r = r * a
    end
    return r
end
N = 3
--powN still computes the 4th power

Here N is a Lua value of type number. When powN is defined, the value of N is looked up in the Lua environment and inlined into the function as an int literal.

Since N is resolved when powN is defined, changing N after powN is compiled will not change the behavior of powN. For this reason, it is strongly recommended that you don't change the value of Lua variables that appear in Terra code once they are initialized.

Of course, a single power function is boring. Instead we might want to create specialized versions of 10 power functions:

local mymath = {}
for i = 1,10 do
    mymath["pow"..tostring(i)] = terra(a : double)
        var r = 1
        for i = 0, i do
            r = r * a
        end
        return r
    end
end

mymath.pow1(2) -- 2
mymath.pow2(2) -- 4
mymath.pow3(2) -- 8

Here we use the fact that in Lua the select operator on tables (a.b) is equivalent to looking up the value in table (a["b"]).

You can call these power functions from a Terra function:

terra doit()
    return mymath.pow3(3) 
end

Let's examine what is happens when this function is compiled. The Terra compiler will resolve the mymath symbol to the Lua table holding the power functions. It will then see the select operator (math.pow3). Because mymath is a Lua table, the Terra compiler will perform this select operator at compile time, and resolve mymath.pow3 to the third Terra function constructed inside the loop. It will then insert a direct call to that function inside doit. This behavior is a form of partial execution. In general, Terra will resolve any chain of select operations a.b.c.d on Lua tables at compile time. This behavior enables Terra to use Lua tables to organize code into different namespaces. There is no need for a Terra-specific namespace mechanism!

Recall how we can include C files:

local c = terralib.includec("stdio.h")

terralib.includec is just a normal Lua function. It builds a Lua table that contains references to the Terra functions that represent calls to (in this case) the standard library functions. We can iterate through the table as well:

for k,v in pairs(c) do
    print(k)
end
--output:
fseek
gets
printf
puts
FILE
...

Terra allows you to use many types of Lua values in Terra functions. Here we saw two examples: the use of a Lua number N into a Terra number, and the use of a Terra function mymath.pow3 in body of doit. Many Lua values can be converted into Terra values at compile time. The behavior depends on the value, and is described in the compile-time conversions section of the API reference.

Scoping

Additionally, you may want to declare a Terra function as a locally scoped Lua variable. You can use the local keyword:

local terra foo()
end

Which is just sugar for:

local foo; foo = terra()
end()

Types and Operators

Terra's type system closely resembles the type system of C, with a few differences that make it interoperate better with the Lua language.

Primitive Types

We've already seen some basic Terra types like int or double. Terra has the usual set of basic types:

• Integers: int int8 int16 int32 int64
• Unsigned integers: uint uint8 uint16 uint32 uint64
• Boolean: bool
• Floating Point: float double

Integers are explicitly sized except for int and uint which should only be used when the particular size is not important. Most implicit conversions from C are also valid in Terra. The one major exception is the bool type. Unlike C, all control-flow explicitly requires a bool and integers are not explicitly convertible to bool.

if 3 then end -- ERROR 3 is not bool
if 3 == 0 then end -- OK! 3 == 0 is bool

You can force the conversion from int to bool using an explicit cast:

var a : bool = [bool](3)

Primitive types have the standard operators defined:

• Arithmetic: - + * / %
• Comparison: < <= > >= == ~=
• Logical: and or not
• Bitwise: and or not ^ << >>

These behave the same C except for the logical operators, which are overloaded based on the type of the operators:

true and false --Lazily evaluated logical and
1 and 3        --Eagerly evaluated bitwise and

Pointers

Pointers behave similarly to C, including pointer arithmetic. The syntax is slightly different to work with Lua's grammar:

var a : int = 1
var pa : &int = &a
@a = 4
var b = @a

You can read &int as a value holding the address of an int, and @a as the value at address a. To get a pointer to heap-allocated memory you can use stdlib's malloc:

C = terralib.includec("stdlib.h")
terra doit()
    var a = [&int](C.malloc(sizeof(int) * 2))
    @a,@(a+1) = 1,2
end

Indexing operators also work on pointers:

a[3] --syntax sugar for @(a + 3)

Pointers can be explicitly cast to integers that are large enough to hold them without loss of precision. The intptr is the smallest integer that can hold a pointer. The ptrdiff type is the signed integer type that results from subtracting two pointers.

Arrays

You can construct statically sized arrays as well:

var a : int[4]
a[0],a[1],a[2],a[3] = 0,1,2,3

In constrast to Lua, Terra uses 0-based indexing since everything is based on offsets. &int[3] is a pointer to an array of length 3. (&int)[3] is an array of three pointers to integers.

The function array will construct an array from a variable number of arguments:

var a = array(1,2,3,4) -- a has type int[4]

If you want to specify a particular type for the elements of the array you can use arrayof function:

var a = arrayof(int,3,4.5,4) -- a has type int[3]
                           -- 4.5 will be cast to an int

Vectors

Vectors are like arrays, but also allow you to perform vector-wide operations:

terra saxpy(a :float,  X : vector(float,3), Y : vector(float,3),)
	return a*X + Y
end

They serve as an abstraction of the SIMD instructions (like Intel's SSE or Arm's NEON ISAs), allowing you to write vectorized code. The constructors vector and vectorof create vectors, and behave similarly to arrays:

var a = vector(1,2,3,4) -- a has type vector(int,3)
var a = vectorof(int,3,4.5,4) -- a has type vector(int,3)
                              -- 4.5 will be cast to an int

Structs

You can create aggregate types using the struct keyword. Structs must be declared outside of Terra code:

struct Complex { real : float; imag : float; }
terra doit()
    var c : Complex
    c.real = 4
    c.imag = 5
end

Unlike C, you can use the select operator a.b on pointers. This has the effect of dereferencing the pointer once and then applying the select operator (similar to the -> operator in C):

terra doit(c : Complex)
    var pc = &c
    return pc.real --sugar for (@pc).real
end

Like functions, symbols in struct definitions are resolved when the struct is defined, and can be linked together using and.

struct C --declaration
struct A {
	b : &B
--and is required since A refers to B
} and struct B { 
	a : &A
	c : &C
--you can mix struct and function
--definitions
} and terra myfunc()
end

struct C { i : int }

They may also contain unnamed members:

var a : struct { float, float }

Unnamed members will be given the names _0, _1, ... _N:

a._0 + a._1

Terra has no explicit union type. Instead, you can declare that you want two or more elements of the struct to share the same memory:

struct MyStruct { 
    a : int; --unique memory
    union { 
        b : double;  --memory for b and c overlap
        c : int;
    } 
}

Anonymous Structs

In Terra you can also create struct types that have no name:

var a : struct { real : float, imag : float } 

These structs are similar to the anonymous structs found in languages like C-sharp.

You can use a struct constructor syntax to quickly generate values that have an anonymous struct type:

var a = { 1,2,3,4 } --has type struct {int,int,int,int}
var b = { a = 3.0, b = 3 } --has type struct { a : double, b : int }

Terra allows you to implicitly convert any anonymous struct to another struct that has a superset of its fields.

struct Complex { real : float, imag : float}
var a : Complex = { real = 3, imag = 1 }

If the anonymous struct has unnamed members, then it they will be used to initialize the fields of the named struct in order:

var b : Complex = {1, 2}

Since constructors like {1,2} are first-class values, they can appear anywhere a Terra expression can appear. This is in contrast to struct initializers in C, which can only appear in a struct declaration.

Function Pointers

Terra also allows for function pointers:

terra add(a : int, b : int) return a + b end
terra sub(a : int, b : int) return a - b end
terra doit(usesub : bool, v : int)
    var a : {int,int} -> int
    if usesub then
        a = sub
    else
        a = add
    end
    return a(v,v)
end

Terra does not have a void type. Instead, functions may return zero arguments:

terra zerorets() : {}
end

Terra Types as Lua Values

Earlier we saw how Terra functions were actually Lua values. The same is true of Terra's types. In fact, all type expressions -- expressions following a ':' in declarations -- are simply Lua expressions that resolve to a type. Any valid Lua expression (e.g. function calls) can appear as a type as long as it evaluates to a valid Terra type:

function Complex(typ)
    return struct { real : typ, imag : typ }
end

terra doit()
    var intcomplex : Complex(int) = {1,2}
    var dblcomplex : Complex(double) = { 1.0, 2.0 }
end

Since types are just Lua expressions they can occur outside of Terra code. Here we make a type alias for a pointer to an int that can be used in Terra code:

local ptrint = &int

terra doit(a : int)
    var pa : ptrint = &a
end

In fact many primitive types are just defined as Lua variables:

_G["int"] = int32
_G["uint"] = uint32
_G["long"] = int64
_G["intptr"] = uint64 --these may be architecture specific
_G["ptrdiff"] = int64

Making types Lua objects enables powerful behaviors such as templating. Here we create a template that returns a constructor for a dynamically sized array:

function Array(typ)
    return terra(N : int)
        var r : &typ = [&typ](C.malloc(sizeof(typ) * N))
        return r
    end
end

local NewIntArray = Array(int)

terra doit(N : int)
    var my_int_array = NewIntArray(N)
    --use your new int array
end

Literals

Here are some example literals:

• 3 is an int
• 3. is a double
• 3.f is a float
• 3LL is an int64
• 3ULL is a uint64
• "a string" or [[ a multi-line long string ]] is an &int8
• nil is the null pointer for any pointer type
• true and false are bool

Expression Lists

In cases where multiple expressions can appear in a list, functions that return multiple will append all of their values to the list if they are the final member of the list. This behavior occurs in declarations, assignments, return statements, and struct initializers.

Here are some examples (adapted from the Lua reference manual):

 f()                -- adjusted to 0 results
 g(f(), x)          -- f() is adjusted to 1 result
 g(x, f())          -- g gets x plus all results from f()
 a,b = f(), x       -- f() is adjusted to 1 result
 a,b,c = x, f()     -- f() is adjusted to 2 results
 a,b,c = f()        -- f() is adjusted to 3 results
 return f()         -- returns all results from f()
 return x,y,f()     -- returns x, y, and all results from f()
 {f()}              -- creates a struct with all results from f()
 {f(), nil}         -- f() is adjusted to 1 result
 {(f())}            -- f adjusted to 1 result

Methods

Unlike languages like C++ or Scala, Terra does not provide a built-in class system that includes advanced features like inheritance or sub-typing. Instead, Terra provides the mechanisms for creating systems like these, and leaves it up to the user to choose to use or build such a system. One of the mechanisms Terra exposes is a method invocation syntax sugar similar to Lua's : operator.

In Lua, the statement:

receiver:method(arg1,arg2)

is syntax sugar for:

receiver.method(receiver,arg1,arg2)

The function method is looked up on the object receiver dynamically. In contrast, Terra looks up the function statically at compile time. Since the value of the receiver expression is not know at compile time, it looks up the method on its type.

In Terra, the statement:

receiver:method(arg1,arg2)

where receiver has type T is syntax sugar for:

T.methods.method(receiver,arg1,arg2)

T.methods is the method table of type T. Terra allows you to add methods to the method tables of named structural types:

struct Complex { real : double, imag : double }
Complex.methods.add = terra(self : &Complex, rhs : Complex) : Complex
    return {self.real + rhs.real, self.imag + rhs.imag}
end

terra doit()
    var a : Complex, b : Complex = {1,1}, {2,1}
    var c = a:add(b)
    var ptra = &a
    var d = ptra:add(b) --also works
end

The statement a:add(b) will normally desugar to Complex.methods.add(a,b). Notice that a is a Complex but the add function expects a &Complex. If necessary, Terra will insert one implicit address-of operator on the first argument of the method call. In this case a:add(b) will desugar to Complex.methods.add(&a,b).

Like the . selection operator, the : method operator can also be used directly on pointers. In this case, the pointer is first dereferenced, and the normal rules for methods are applied. For instance, when using the : operator on a value of type &Complex (e.g. ptra), it will first insert a dereference and desugar to Complex.methods.add(@a,b). Then to match the of add, it will apply the implicit address-of operator to get Complex.methods.add(&@a,b). This allows a single method definition to take as an argument either a type T or a pointer &T, and still work when the method is called on value of type T or type &T.

To make defining methods easier, we provide a syntax sugar.

terra Complex:add(rhs : Complex) : Complex
	...
end

is equivalent to

terra Complex.methods.add(self : &Complex, rhs : Complex) : Complex
    ...
end

Terra also support metamethods similar to Lua's operators like __add, which will allow you to overload operators like + on Terra types, or specify custom type conversion rules. See the API reference on structs for more information.

Lua-Terra Interaction

We've already seen examples of Lua code calling Terra functions. In general, you can call a Terra function anywhere a normal Lua function would go. When passing arguments into a terra function from Lua they are converted into Terra types. The current rules for this conversion are described in the API reference. Right now they match the behavior of LuaJIT's FFI interface. Numbers are converted into doubles, tables into structs or arrays, Lua functions into function pointers, etc. Here are some examples:

struct A { a : int, b : double }

terra foo(a : A)
    return a.a + a.b
end

assert( foo( {a = 1,b = 2.3} )== 3.3 )
assert( foo( {1,2.3} ) == 3.3)
assert( foo( {b = 1, a = 2.3} ) == 3 )

More examples are in tests/luabridge*.t.

It is also possible to call terra functions from Lua. Again, the translation from Terra objects to Lua uses LuaJITs conversion rules. Primtive types like double will be converted to their respective Lua type, while aggregate and derived types will be boxed in a LuaJIT ctype that can be modified from Lua:

function add1(a)
    a.real = a.real + 1
end
struct Complex { real : double, imag : double }
terra doit()
    var a : Complex = {1,2}
    add1(&a)
    return a
end
a = doit()
print(a.real,a.imag) -- 2    1
print(type(a)) -- cdata

The file tests/terralua.t includes more examples. The file tests/terraluamethod.t also demonstrate using Lua functions inside the method table of a terra object.

since we cannot determine the Terra types that function will return, Lua functions do not return values to Terra functions by default. To convert a Lua function into a Terra function that does return a value, you first need to cast it to a Terra function type:

function luaadd(a,b) return a + b end
terraadd = terralib.cast( {int,int} -> int, luaadd)
 
terra doit()
	return terraadd(3,4)
end

Meta-programming

In this guide we've already encountered instances of meta-programming, such as using a Lua loop to create an array of Terra pow functions. In fact, Terra includes several operators that it make it possible to generate any code at runtime. For instance, you can implement an entire compiler by parsing an input string and constructing the Terra functions that implement the parsed code.

The operators we provide are adapted from multi-stage programming. An escape allows you to splice the result of a Lua expression into Terra. A quote allows you to generate a new Terra statement or expression which can then be spliced into Terra code using an escape. Symbol objects allow you to create unique names at compile time. Finally, a macro can be used like a function call in Terra code but will be evaluated at compile-time. We'll look at each of these operator in detail.

Escapes

Escapes allow you to splice the result of a Lua expression into Terra code. Here is an example:

function get5()
	return 5
end
terra foobar()
	return [ get5() + 1 ]
end
foobar:printpretty()
> output: 
> foobar0 = terra() : {int32}
> 	return 6
> end

When the function is defined, the Lua expression inside the brackets ([]) is evaluated to the Lua value 6 which is then used in the Terra code. The Lua value is converted to a Terra value based on the rules for compile-time conversions in the API reference (e.g. numbers are converted to Terra constants, global variables are converted into references to that global).

Escapes can appear where any expression or statement normally appears. When they appear as statements or at the end of en expression list, multiple values can be spliced in place by returning a Lua array:

terra return123()
	--escape appends 2 values:
	return 1, [ {2,3} ] 
end

You can also use escapes to programmatically choose fields or functions:

local myfield = "foo"
local mymethod = "bar"
terra fieldsandfunctions()
	var fields = myobj.[myfield] 
	var methods = myobj:[mymethod]()
end

Lua expressions inside an escape can refer to the variables defined inside a Terra function. For instance, this example chooses which variable to return based on a Lua parameter:

local choosefirst = true
local function choose(a,b)
	if choosefirst then
		return a
	else
		return b
	end
end
terra doit(a : double)
	var first = C.sin(a)
	var second = C.cos(a)
	return [ choose(first,second) ]
end

Since Lua and Terra can refer to the same set of variables, we say that they share the same lexical scope.

What values should first and second have when used in an escape? Since escapes are evaluated when a function is defined, and not when a function is run, we don't know the results of the sin(a) and cos(a) expressions when evaluating the escape. Instead, first and second will be symbols, an abstract data type representing a unique name used in Terra code. Outside of a Terra expression, they do not have a concrete value. However, when placed in a Terra expression they become references to the original variable. Going back to the example, the function doit will return either the value of C.sin(a) or C.cos(a) depending on which symbol is returned from the choose function and spliced into the code.

Previously, we have seen that you can use Lua symbols directly in the Terra code. For example, we looked at this powN function:

local N = 4
terra powN(a : double)
    var r = 1
    for i = 0, N do
        r = r * a
    end
    return r
end

This behavior is actually just syntax sugar for an escape expression. In Terra, any name used in an expression (e.g. a or r) is treated as if it were an escape. Here is the same function de-sugared:

local N = 4
terra powN(a : double)
    var r = 1
    for i = 0, N do
        r = [r] * [a]
    end
    return [r]
end

In this case [a] will resolve to the value 4 and then be converted to a Terra constant, while [r] will resolve to a symbol and be converted to a reference to the variable definition of r on the first line of the function.

The syntax sugar also extends to field selection expressions such as a.b.c. In this case, if both a and b are Lua tables, then the expression will de-sugar to [a.b.c]. For instance, the call to C.sin and C.cos are de-sugared to [C.sin] and [C.cos] since C is a Lua table.

Quotes

A quote allows you to generate a single Terra expression or statement outside of a Terra function. They are frequently used in combination with escapes to generate code. Quotes create the individual expressions and escapes are used stitch them together.

function addone(a)
	--return quotation that
	--represents adding 1 to a    
	return `a + 1 
end
terra doit()
	var first = 1
	--call addone to generate
	--expression first + 1 + 1
	return [ addone(addone(first)) ]
end

If you want to create a group of statements rather than expressions, you can using the quote keyword:

local printit = quote
	C.printf("a quotestatement")
end

terra doit()
	--print twice
	printit
	printit
end

When a variable is used in an escape, it is sometimes ambiguous what value it should have. For example, consider what value this function should return:

function makeexp(arg
    return quote 
        var a = 2
        return arg + a
    end)
end
terra client()
    var a = 1;
    [ makeexp(a) ];
end

The variable name a is defined twice: once in the function and once in the quotation. A reference to a is then passed the makeexp function, where it is used inside the quote after a is defined. In the return statement, should arg have the value 1 or 2? If you were using C's macro preprocessor, the equivalent statement might be something like

#define MAKEEXP(arg) \
    int a = 2; \
    return arg + a; \

int scoping() {
    int a = 1;
    MAKEEXP(a)
}

In C, the function would return 4. But this seems wrong -- MAKEEXP may have been written by a library writer, so the writer of scoping might not even know that a is used in MAKEEXP. This behavior is normally call unhygienic since it is possible for the body of the quotation to accidentally redefine a variable in the expression. It makes it difficult to write reusable functions that generate code and is one of the reasons macros are discouraged in C.

Instead, Terra ensures that variable references are hygienic. The reference to a in makeexp(a) refers uniquely to the definition of a in the same lexical scope (in this case, the definition of a in the client function). This relationship is maintained regardless of where the symbol eventually ends up in the code, so the scoping function will correctly return the value 3.

This hygiene problem occurs in all languages that have meta-programming. Wikipedia has more discussion. By maintaining hygiene and using lexical scoping, we guarantee that you can always inspect a string of Terra code and match variables to their definitions without know how the functions will execute.

Dynamically Generated Symbols

For the most part, hygiene and lexical scoping are good properties. However, you may want to occasionally violate lexical scoping rules when generating code. For instance, you may want one quotation to introduce a local variable, and another separate quotation to refer to it. Terra provides a controlled way of violating lexical scoping using the symbol() function, which returns a unique variable name (a symbol) each time it is called (this is the Terra equivalent of Common Lisp's gensym). Here is an example that creates a new symbol, defines the symbol in one quotation and then uses it in another.

local a = symbol()

defineA = quote
        var [a] = 3
    end

twiceA = `2*a

terra doit()
    defineA
    return twiceA
end

The symbol function can also take a type as an argument symbol(int). This has the same effect as when you write var a : int in a declaration. It is optional when the type of the definition can be inferred (e.g. when it is local variable with an initializer), but required when it cannot be inferred (e.g. when it is a parameter to a function).

Notice that the declaration of the symbol uses the escape [a] in place of a. Using just a would make a local variable with name a that is not in scope outside of that quotation. In this context, the escape instructs the Terra compiler to parse that part as a Lua expression, evaluate it, and drop the result in place. In this case, the result of evaluation a is the symbol generated by the symbol() function. Similarly the reference to a in the expression 2*a will evaluate to the same symbol object. If we had omitted the escape, then we would receive a compilation error reporting that 2*a refers to an undefined variable.

A list of symbols can also be spliced onto the end of parameter lists to generate functions with a configurable number of arguments:

local rest = {symbol(int),symbol(int)}

terra doit(first : int, [rest])
    return first + [rest[1]] + [rest[2]]
end

Macros

By default, when you call a Lua function from Terra code, it will execute at runtime, just like a normal Terra function. It is sometimes useful for the Lua function to execute at compile time instead. Calling the Lua function at compile-time is called a macro since it behaves similarly to macros found in Lisp and other languages. You can create macro using the function macro which takes a normal Lua function and returns a macro:

local times2 = macro(function(ctx,tree,a)
    return `a + a
end)

terra doit()
    var a = times2(3)
    -- a == 6
end

Unlike a normal function, which works on Terra values, the arguments to Terra macros are passed to the macro as quotes.

The first argument to every macro is the compilation context ctx. It can be used to report an error if the macro doesn't apply to the arguments given, and is needed in certain API calls used in macros. The second argument to every macro (tree) is the AST node in the code that represents the macro call. It is typically used as the location at which to report an error in a macro call. The following code will cause the compiler to emit an error referring to the macro call with given error message:

ctx:reporterror(tree, "something in the macro went wrong")

The remaining arguments to the macro are the AST nodes of the arguments to the macro function.

Since macros take quotes rather than values, they have different behavior than function calls. For instance:

var c = 0
terra up()
    c = c + 1
    return c
end

terra doit()
    return times2(up()) --returns 1 + 2 == 3
end

The example returns 3 because up() is evaluated twice

Some built-in operators are implemented as macros. For instance the sizeof operator just inserts a special AST node that will calculate the size of a type:

sizeof = macro(function(ctx,tree,typ)
    return terralib.newtree(tree,{ kind = terra.kinds.sizeof, 
                                   oftype = typ:astype()})
end) 

terra.newtree creates a new node in this AST. For the most part, macros can rely on code quotations to generate AST nodes, and only need to fallback to explicitly creating AST nodes in special cases.

Macros can also be used to create useful patterns like a C++ style new operator:

new = macro(function(ctx,tree,typquote)
    local typ = typquote:astype()
    return `[&typ](C.malloc(sizeof(typ)))
end)

terra doit()
    var a = new(int)
end

You may be wondering why Terra includes both macros and escapes. They both allow you to splice Terra code into other expressions, and in some cases you can use either a macro or an escape to accomplish the same purpose. Since macros look like function calls, they are normally used when it is not important for the end-user to know that the functionality is implemented by generating code. For instance, in myobj:mymethod(arg), mymethod can be implemented as a macros. Furthermore, while escapes are evaluated when a function is defined (that is, when the surround Lua code executes), macros are run when a function is compiled, which only happens when a function is actually called. This means that macros have access to the types of expressions via the myquote:gettype() method call.

More Information

More details about the interaction of Terra and Lua can be found in the API reference. The best place to look for more examples of Terra features is the tests/ directory, which contains the set of up-to-date languages tests for the implementation. The tests/libs folder contains some examples of meta-programming such as class systems.

If you are interested in the implementation, you can also look at the source code. The compiler is implemented as a mixture of Lua code and C/C++. Passing the -v flag to the interpreter will cause it to give verbose debugging output.

• lparser.cpp is an extended version of the Lua parser that implements Terra parsing. It parsers Terra code, building the Terra AST for Terra code, while passing the remaining code to Lua (use -vv to see what is passed to Lua).

• terralib.lua contains the Lua infrastructure for the Terra compiler, which manages the Terra objects like functions and types. It also performs type-checking on Terra code before compilation.

• tcompiler.cpp contains the LLVM-based compiler that translates the Terra AST into LLVM IR that can then be JIT compiled to native code.

• tcwrapper.cpp contains the Clang-based infrastructure for including C code.

• terra.cpp contains the implementation of Terra API functions like terra_init

• main.cpp contains the Terra REPL (based on the Lua REPL).