Declarations introduce new names for entities in the program. Declarable entities include

  • activities
  • functions
  • types
  • data storage

In the future more of such entities may be added to the language such as clocks and physical units.

Every declaration exists in a lexical scope. Scopes define the visibility of a name. The top-level or file scope is the most global scope in Blech. Functions, activities and types are defined here. Entities in the top-level scope are visible everywhere in the file. Every block introduces a sub-scope. For example, a function body is a local scope. Variables defined in this scope are visible in this function but not outside of it. Composite statements, such as the repeat loop introduce their statement-local scope. This allows to introduce a variable that is visible during the iteration through this loop and not outside of it.

In Blech, declarations not only introduce a name but also define what this name represents. For example, a function declaration will specify a function body. That is the block of statements that are executed when this functions is called. The definition may refer to names that were previously declared.

In the following we discuss the various entities that can be defined in a Blech program.


Data declarations consist of an access and placement qualifier, an identifier, a data type and an initialisation.

DataDeclaration ::= Qualifier Identifier ":" Type "=" Init

A variable declaration:

var x: int32 = 17

The type is either a built-in type or a name of a user defined type. Built-in types are discussed in the Types chapter . Type declarations are discussed below .

Initialisers are expressions that evaluate to a value that matches this declaration’s data type. Expressions (including literals) are discussed in the Expressions chapter .


There are four possible qualifiers.

Qualifier ::= "const" | "param" | "let" | "var"

const qualified data is relevant for the compilation phase only and does not have any representation in memory at run time. Typical use cases for const are sizes of data structures, addresses in memory, … In C, these are represented by #define macros, which, too, have no representation after the preprocessor finishes. Hence using const on arrays (or structs containing arrays), albeit possible, is not advisable and produces inefficient code because temporary variables have to be created (and filled) at runtime for every (non-constant) access to these arrays. Instead consider param.

param qualified data has a representation in memory at run time but cannot be changed by the running program for its entire lifetime (it can only be reflashed). Typical use cases for param are characteristics maps or other immutable lookup data structures. The name “param” indicates that such data is a parameter of the final compilation result. Tools exist to customise such parameters in a binary file directly in order to adapt the given software to a variant of a product. Like with const data, the param value must be initialised with constant expressions. You cannot write the following

function f (input: int32)
   param x: int32 = input // error!

This is because the value of input is determined at runtime but x must be initialised at compile time.

const and param data may be declared at top level as well as inside functions or activities. The other two qualifiers let and var indicate local data and may only be used inside functions or activities.

let declares immutable data in the control flow of a program. Finally, var declares the usual mutable variable. Initialisation may be omitted for mutable variables. In this case the Blech compiler will automatically initialise the variable with its type’s default value. The type annotation may be omitted if the type can be unambiguously determined from the initialisation expression.


const LEN: int32 = 8
param lut: [LEN]float64 = {1.0, 0.5, 0.25, 0.125} // LEN is constant and may be used here
                                                  // the array literal will be filled with additional 0's up to length LEN.
function f()
    let i = LEN - 7 // i is deduced to be int32
    var x = lut[i] // x is deduced to be float64
                   // and equals 0.5 in this case

User defined types

The programmer may define a data structure using the struct keyword. See the chapter on struct types for more details.


Blech discerns two type of subprograms: activity and function. Their behaviour is different. Activities must pause at least once whereas functions need to terminate within one reaction. Functions are therefore called “instantaneous”. The precise differences will be worked out in the chapter on Blech statements. From a syntactic declaration point of view there is hardly any difference.

ProgramDeclaration ::= ["singleton"] ProgramType Identifier ParamList [ParamList] ["returns" Type] StmtBlock "end"
ProgramType        ::= "function" | "activity"
ParamList          ::= "()" | "(" ParamDeclaration ("," ParamDeclaration)* ")"
ParamDeclaration   ::= Identifier ":" Type

There are two parameter lists. The first lists declares formal parameters that may only be read (like let variables), the second list declares formal parameters that may be both read and written (like var variables). In particular the two lists are useful for activities which, in every reaction, receive a list of read-only inputs, perform some calculation and set the list of read-write outputs. We will therefore often refer to these two parameter lists as “input list” and “output list”.

The programming model is that all variables are passed by reference (even though in reality the compiler will optimise this into by-value for simple value typed inputs).


function add (x: int32, y: int32) returns int32
   return x + y

activity A (in: int32)(out: int32)
      out = add(in, out)
      await true

The example above is a valid Blech program that sums all inputs over all time steps. Note that add omits an output list and A does not declare any return type. We call functions or activities that do not return anything “void” but unlike C we do not have a void type in the language.

The @[EntryPoint] annotation tells the compiler that A is the main program. Every Blech program must have precisely one entry point activity. Note however, that a Blech program may consist of several files, one program file and several (sub-)modules. See the modules chapter for details.

The singleton keyword is optional and may be used to indicate that there may exist only one instance of this subprogram in a concurrent context. For example, this is useful to indicate early on in the development phase that an activity will have some interaction with the external environment. The caller of a singleton callee automatically becomes a singleton, too.

External Declarations

Sometimes it is useful to access global variables or functions of a C program. This allows for example to make use of existing libraries. Such variables and functions are external from the point of view of a Blech program. Annotations are required to tell the compiler how to code-generate access to these external entities.

Formally, we have the following syntax.

ExternFunctionDeclaration ::= "extern" ["singleton"] "function" Identifier ParamList [ParamList] [returns Type]
ExternDataDeclaration     ::= "extern" Qualifier Identifier ":" Type

Obviously, external functions have no body and external variables cannot be initialised. As before, external functions may be characterised as singleton which means such a function may not be called concurrently with another instance of itself. This is useful when the external function to be called is not a pure function because it either returns a volatile value or has some effect on the environment. Invoking multiple instances of such a function concurrently would violate the synchrony assumptions and lead to unexpected results.

External declarations additionally require annotations which we introduce by example below.

External constants

In C, constant values may be defined using macros or const variables. In order to make these values available in Blech, external constants may be declared. External constant declarations may appear in any scope.

@[CConst (binding = "PI", header = "math.h")]
extern const pi: float64
@[CParam (binding = "characteristics", header = "magic.h")]
extern param map: [10]float32

Both Blech qualifiers const and param are supported. They require a CConst or a CParam annotation respectively. However they have more of a documentation character rather than any functional difference. Both will evaluate whatever expression is given in the binding at runtime. This is the reason why external constants cannot be used for constant expression evaluation in Blech – their value is unknown at compile time. While you can, for example, use a Blech constant to parametrise an array length, you cannot do so using an external constant.

The binding annotation attribute may contain any string which is a valid right hand side of a C macro. See below for more details.

By design, the Blech compiler generates C code that links with other C code but at no point in time does the Blech compiler “look into” C header or implementation files, nor does it try to evaluate any C-bindings.

Local external variables

The aforementioned constants may be declared in local scopes as well. Additionally, local Blech variables that link to external global variables may be declared inside activities (but not in functions).

Access to external variables is useful to keep interfaces slim. That is you do not need to pass all data into the entry point activity and down the call chain to the piece of code that actually needs this data and then propagate the results back up this chain to the entry point to communicate the updated values to the environment. These variables follow the same rules as the usual activity-local variables.

Read-only external variables are annotated with the CInput annotation.

@[CInput (binding = "PIN_7", header = "head.h")]
extern let isButtonPressed: bool

This example assumes there is either a C macro or a C variable PIN_7 that returns a volatile boolean value indicating a button press.

The declaration creates a local variable inside the enclosing activity. It serves as a copy-in buffer. When the activity starts a reaction the value of PIN_7 is copied into isButtonPressed. Within the Blech program we can only access the buffer isButtonPressed and thereby have the guarantee that the value does not change during the reaction. This corresponds to the semantics of activity input parameters.

An activity that declares an immutable external variable does not become a singleton. Concurrent instances may exist but they may contain different values for the same external variable if it is volatile.

Read-write external variables are annotated with the COutput annotation.

@[COutput (binding = "PIN_7", header = "head.h")]
extern var isButtonPressed: bool

Here at the beginning of a reaction the value of PIN_7 is copied in. During a reaction the variable isButtonPressed can be modified as usual. At the end of the reaction the value of isButtonPressed is copied out to PIN_7. This guarantees a stable output behaviour. Intermediate changes to the local variable isButtonPressed are not observable by the environment.

The prev operator may be used on external variables. It returns the value that the variable held at the end of the previous reaction. This behaviour corresponds to using prev on normal local variables but there is a subtle difference. External variables may be changed by the environment.

@[COutput (binding = "PIN_7", header = "head.h")]
extern var isButtonPressed: bool
isButtonPressed = true
await cond // some boolean condition
var x = prev isButtonPressed // is x == true?

If cond is true immediately in the next reaction then x will be set to true. In general, however we do not know how many reaction it will take until cond becomes true. Yet in every reaction the copy-in and copy-out mechanisms will update the isButtonPressed buffer. If the environment does not change PIN_7 then surely x will be true. But, in general, we cannot assume this.

An activity that declares a mutable external variable automatically becomes a singleton. Concurrent instances lead to a write-write conflict and compilation is rejected.

External functions

There are two ways to link to external functions in Blech.

  1. Via direct binding a to function name declared in an .h file
  2. Via a wrapper to be implemented in some .c file.

In the first case we annotate the call to the C function and the file wherein this function is declared. The call must list the parameters using a $i notation. In this way, we can map the input and output parameters of the Blech prototype to the positions of the C function.

@[CFunction (binding = "ceil($1)", header = "math.h")]
extern function ceiling(i: float64) returns float64

Inside the Blech program this function is now available through name ceiling.

In the second case we annotate which file we intend to implement the C function in. Actually this information is irrelevant for the Blech compilation itself. However, it may become useful in the future once a build system can make sense of these annotations and automatically detect which files are required for the compilation of the whole project.

@[CFunction (source="impl.c")]
extern function myCFunction(i: float64) returns float64

Assume the above declaration is written in a Blech file called MyFile.blc, then the code generator will produce a header file MyFile.h with the following code:

// extern functions to be implemented in C
blc_float64 blc_MyFile_myCFunction (const blc_float64 blc_i);

It is up to the C programmer now to include this header in his implementation file impl.c and provide an actual definition of this function.

Remarks on caveats when interfacing with C


Blech has no representation of C types. It requires that the C implementation matches the Blech types. This is usually straightforward for simple types. If there is no one-to-one correspondence between types then a wrapper has to be implemented in C that marshals the data between Blech and the actual C function to be called.

Parameter lists

In Blech, functions have two parameter lists as explained above. The Blech compiler ensures that inputs will only be read. However the Blech compiler has no chance to check that the external code adheres to this contract.

For example, say we have an external function that takes an array of length 10 and sorts it in-place. The correct binding would look something like this:

@[CFunction (binding = "sort($1)", header = "utils.h")]
extern function sort()(arr: [10]int32)

In this way, the Blech compiler knows that sort will modify the given array. When calling this function in a concurrent context the compiler will prevent write-write conflicts and read-write cycles on the array.

However, the programmer could erroneously declare the same function as follows:

@[CFunction (binding = "sort($1)", header = "utils.h")]
extern function sort(arr: [10]int32)

The code will compile all the same but the causality guarantees are gone because the Blech compiler relies on the assumption that the array will only be read and not modified. At runtime the program may then exhibit unexpected behaviour.


The singleton annotation is a help to the Blech programmer but does not completely prevent concurrent calls to functions with conflicting effects. For example:

@[CFunction (binding = "foo", header = "head.h")]
extern singleton function doA() 
@[CFunction (binding = "foo", header = "head.h")]
extern singleton function doB() 

/* ... somewhere in an activity scope ... */

This example is a valid Blech program because two different singleton functions are called. This is allowed. However the annotation points to the same C function which is obviously a problem. While a linter could in principle check for this particular mistake there are many more possibilities to specify bindings to functions which will have conflicting effects when called concurrently. It is up to the programmer to know what are the effects of the external functions to be called and to avoid scenarios such as the one above.

Binding strings

The binding part of an annotation may contain any C code which is a valid right hand side of a macro definition. The binding may be given as a single-line string (enclosed in ") or as a multi-line string (enclosed in """). When indenting multi-line strings make sure the indentation is consistent for all lines of the string. This means a two line string where the first line is indented by four spaces and a tab and the second line is indented by a tab and four spaces may look indented properly in your editor but the compiler will complain about being unable to determine the correct indentation. If you use any tabs at all, make sure all tabs appear before any other character (including whitespace) in every line. Strings may contain escape sequences \a, \b, \f, \n, \r, \t, \v, \\, \', \". Furthermore unicode characters may be used given by \u{codepoint} where codepoint is a hexadecimal number.

Last modified May 4, 2021: drafting the module chapter (fa3db01)