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Lambdas

A lambda consists of a sequence of statements that can be bound to a variable. The variable can be copied and called as needed. Unlike most languages, Pyrope only supports anonymous lambdas. The reason is that without it lambdas would be assigned to a namespace. Supporting namespaces would avoid aliases across libraries, but Pyrope allows different versions of the same library at different parts of the project. This will effectively create a namespace alias. The solution is to not have namespaces but relies upon variable scope to decide which lambda to call.

Observation

Allowing multiple version of the same library/code is supported by Pyrope. It looks like a strange feature from a software point of view, but it is common in hardware to have different blocks designed/verified at different times. The team may not want to open and modernize a block. In hardware, it is also common to have different blocks to be compiled with different compiler versions. These are features that Pyrope enables.

Pyrope divides the lambdas into three categories: comb, pipe, and mod.

  • comb is pure combinational logic. The outputs are purely a function of the inputs — no registers, no state, no cycle-level side effects. Any external call inside a comb can only affect debug statements (e.g., puts), not synthesizable code. comb can use ref arguments to modify tuples; ref is equivalent to having the argument as both input and output, which is still purely combinational. comb resembles pure functions in normal programming languages.

  • pipe is a Moore machine — every output goes through at least one flop. The latency is written as an argument to the keyword: pipe[3] foo(...) is fixed 3-cycle, pipe[1..=3] foo(...) lets the caller pick within a range, and bare pipe foo(...) leaves the latency fully flexible for the caller to specify via await[N] at the call site. The tool may retime logic for performance, but the behavior is equivalent to a comb with N flops appended at the outputs. pipe can use reg for internal storage; besides storage, it behaves like a comb with pipelined outputs.

  • mod has no constraints on registers or outputs. It can be combinational, Mealy, Moore, or a pipeline orchestrator. When a mod calls pipe lambdas and needs to align their outputs with other signals, it uses the await[N] declaration modifier and the @[N] cycle type check (these constructs used to belong to the separate flow category, which has been merged into mod).

Methods are comb/pipe/mod lambdas that have self as the first argument, which allows operating on tuples.

comb add(a, b) -> (result) {  // Same as const add = comb(a, b) -> (result)
  result = a + b
}

pipe[3] multiply(a, b) -> (result) {          // fixed 3-cycle latency
  result = a * b
}

pipe[1..=3] add_pipe(a, b) -> (result) {      // caller picks 1-3 cycles
  result = a + b
}

pipe flexible_mul(a, b) -> (result) {         // bare: caller picks via await[N]
  result = a * b
}

pipe mul(a, b) -> (c) { c = a * b }
pipe add(a, b) -> (c) { c = a + b }

mod multiply_add(in1, in2) -> (out) {
  await[3] tmp      = mul(in1, in2)
  await[3] in1_d    = in1
  await[1] out@[4] = add(tmp@[3], in1_d@[3])
}

mod accum(in1, in2) -> (out) {
  reg total = 0                             // mod can use reg
  await[3] tmp = mul(in1, in2)
  total.[defer] = add(total, tmp@[3])
  out = total
}

mod counter(enable) -> (reg count) {
  count += 1 when enable
}

mod add_reg(a, b) -> (reg result) {
  result = a + b
}

Declaration

There are two interchangeable forms for declaring a lambda. The kind-first declaration form is preferred — it matches the rest of Pyrope's grammar, where every declaration starts with a kind keyword (const/mut/reg for data, comb/pipe/mod for lambdas):

comb get_five() -> (v) { v = 5 }              // kind-first form

Both forms are legal at top level, inside tuple literals, and inside code blocks. The kind-first form is shorter, reads like a method declaration in most languages, and lets an agent scan for all lambda declarations with a single pattern. Lambdas are always immutable.

Only anonymous lambdas are supported — there is no global scope for functions, procedures, or modules. The only way for a file to access a lambda is to have access to a local variable with a definition or to "import" a variable from another file.

const a_3 = { 3 }             // just scope, not a lambda. Scope is evaluated now
comb a_lambda() -> (v) { v = 4 }   // kind-first form

comb get_five() -> (v) { v = 5 }   // public lambda that can be imported by other files

const x = a_3()             // error: explicit call not possible in scope
const x = a_lambda()        // OK, explicit call needed when no arguments

cassert a_3 == 3
type a_lambda_type = comb()->(v)
cassert a_lambda equals a_lambda_type
cassert a_lambda() == 4

The lambda definition has the following fields:

[GENERIC] [COMPTIME] [INPUT] [-> OUTPUT] |

  • GENERIC is an optional comma separated list of names between < and > to use as generic types in the lambda.

  • COMPTIME has the optional list of explicit comptime parameters for the lambda. Each entry is a typed declaration (e.g., n:int) or a typed declaration with a default (e.g., n:int=1). Defaults may refer to visible comptime bindings from the enclosing scope. Callers can override any comptime parameter at the call site using the same [...] slot (foo[N](args)).

  • Lambdas do not have an explicit capture list. Visible comptime bindings from enclosing scopes, including imports and comptime const declarations, are available lexically inside the lambda body and signature. The compiler records those references as explicit comptime dependencies of the lambda; no capture syntax is written by the programmer. Runtime const, mut, and reg declarations from enclosing lambda scopes are not visible inside a nested lambda unless passed as normal inputs or stored in an explicit tuple/object that the lambda receives.

  • INPUT has a list of inputs allowed with optional types. () indicates no inputs. (...args) allow to accept a variable number of arguments.

  • OUTPUT has a list of outputs allowed with optional types. () indicates no outputs.

Dispatch between alternative lambdas is always explicit at the call site using if/elif chains. Pyrope does not have a where clause on lambda declarations (an earlier design did); this keeps the call flow visible and locally readable.

comb add1(...x) { x[0] + x[1] + x[2] }      // no IO specified
comb add2(a, b, c) { a + b + c }            // constrain inputs to a,b,c
comb add3(a, b, c) { a + b + c }            // same
comb add4(a:u32, b:s3, c) { a + b + c }     // constrain some input types
comb add5(a, b, c) -> (x:u32) { a + b + c } // constrain result to u32
comb add6(a, b, c) -> (result) { a + b + c } // constrain result to be named result
comb add7(a, b:a, c:a) { a + b + c }        // constrain inputs to have same type
comb add8<T>(a:T, b:T, c:T) { a + b + c }   // same

// To overload, declare each lambda separately and gather them:
const add = [add1, add2, add3, add4, add5, add6, add7, add8]

const x = 2
comb addx1(a) { x + a }           // error: x is runtime, not visible in lambda

/// Visible comptime bindings are available lexically:
comptime const Scale = 2
comb addx2(a) { Scale + a }       // OK: Scale is comptime

/// Imports are comptime aliases:
const lib = import("lib.math")
comb is_add(op:lib.OpType) { op == lib.AddOp }

/// Comptime parameters can be declared with a type and/or default:
comb scale[n:int=1](a) { n * a }
cassert scale(5) == 5           // uses default n=1
cassert scale[10](5) == 50      // override n=10 at the call site

/// Defaults can use visible comptime bindings:
comptime const DefaultScale = 3
comb scale_default[n:int=DefaultScale](a) { n * a }
cassert scale_default(4) == 12
cassert scale_default[100](4) == 400

mut y = (
  mut val:u32 = 1,
  comb inc1(ref self) { self.val = u32(self.val + 1) }
)

comb my_log::[debug](...inp) {
  print "logging:"
  for i in inp {
    print " {}", i
  }
  puts
}

comb f<X>(a:X, b:X) { a + b }   // enforces a and b with same type
cassert f(u22(33), u22(100)) == 133

my_log(a, false, x + 1)

Argument naming

Input arguments must be named. E.g: fcall(a=2,b=3) There are the following exceptions that avoid naming arguments:

  • If the type system can distinguish between unnamed arguments (no ambiguity)

  • If there is an argument/call match. The calling variable name has the same as an argument

  • If the argument is a single letter, and there is no name match, only position is used

  • self does not need to be named (first argument position)

There are several rules on how to handle arguments.

  • Every lambda call requires parentheses. foo(), foo(1,2), and x.bar(y) are the only forms. There is no "drop parens after newline" or "drop parens after a pipeline operator" sugar. This keeps every call site unambiguously identifiable.

  • Calls use the Uniform Function Call Syntax (UFCS) when self is defined as the first argument. (a,b).f(x,y) is equivalent to f((a,b),x,y).

Pyrope uses a Uniform Function Call Syntax (UFCS) when the first argument is self. It resembles Nim or D UFCS but it can be different from the order in other languages.

comb div(self, b) { self / b }      // named input tuple
comb div2(...x) { x[0] / x[1] }     // unnamed input tuple

comb noarg() { 33 }                 // explicit no args

cassert 33 == noarg()             // () always required, even for no-arg calls

assert noarg                      // error: `noarg()` needed for calls

a = div(3, 4, 3)         // error: div has 2 inputs
b = div(self=8, b=4)     // OK, 2
d = (self=8).div(b=2)    // OK, 4
d = (8).div(b=2)         // OK, 4 . self does not need to be named
d = 8.div(2)             // OK, single character inputs no need to be named

h = div2(8, 4, 3)        // OK, 2 (3rd arg is not used)
i = 8.div2(4, 3)         // error: no self in div2

n = div((8, 4), 3)       // error: (8,4)/3 is undefined
o = (8, 4).div2(1)       // error: (8,4)/1 is undefined

The UFCS allows to have lambdas to call any tuple, but if the called tuple has a lambda defined with the same name a compile error is generated. Like with variables, Pyrope does not allow lambda call shadowing. Polymorphism is allowed but only explicit one as explained later.

mut tup = (
  comb f1(self) { 1 }
)

comb f1(self) { 2 } // error: f1 shadows tup.f1
comb f1() { 3 }     // OK, no self

assert f1() != 0         // error: missing argument
assert f1(tup) != 0      // error: f1 shadowing (tup.f1 and f1)
assert 4.f1() != 0       // error: f1 can be called for tup, so shadow
assert tup.f1() != 0     // error: f1 is shadowing

comb xx(self:tup) { self.f1() } // OK, explicit input restricts scope for f1
cassert xx(tup) == 1

cassert (4:tup).f1() == 1
cassert 4.f1() == 3       // UFCS call
cassert tup.f1() == 1

The keyword self is used to indicate that the function is accessing a tuple. self is required to be the first argument. If the method modifies the tuple contents, a ref self must be passed as input. Since ref is equivalent to having the argument as both input and output, comb can use ref and still be purely combinational. Use mod only when the method needs registers or cycle-level state.

mut tup2 = (
  mut val:u8 = 0sb?,
  comb upd(ref self) { sat self.val += 1 },
  comb calc(self) { self.val }
)

A lambda call always uses parentheses (foo() or foo(1, 2)). The only exception is a variable with a getter method — reading the variable (without parens) implicitly invokes the getter.

no_arg_fun()     // parentheses always required
arg_fun(1, 2)    // parenthesis required

mut intercepted:(
  mut field:u32,
  comb getter(self) { self.field + 1 },
  comb setter(ref self, v) { self.field = v }
) = 0

cassert intercepted == 1  // will call getter method without explicit call
cassert intercepted.field == 0

Pass by reference

Pyrope is an HDL, and as such, there are not memory allocation issues. This means that all the arguments are pass by value and the language has value semantics. In other words, there is not need to worry about ownership or move/forward semantics like in C++/Rust. All the arguments are always by value. Nevertheless, sometimes is useful to pass a reference to an array/register so that it can be updated/accessed on different lambdas.

Pyrope arguments are by value, unless the ref keyword is used. Pass by reference is needed to avoid the copy by value of the function call. Unlike non-hardware languages, there is no performance overhead in passing by value. The reason for passing as reference is to allow the lambda to operate over the passed argument. If modified, it behaves like if it were an implicit output. This is quite useful for large objects like memories to avoid the copy.

The pass by reference behaves like if the calling lambda were inlined in the caller lambda while still respecting the lambda scope. The ref keyword must be explicit in the lambda input definition but also in the lambda call. The lambda outputs can not have a ref modifier.

No logical or arithmetic operation can be done with a ref. As a result, it is only useful for lambda input arguments.

comb inc1(ref a) { a += 1 }

const x = 3
inc1(ref x)       // error: `x` is immutable but modified inside inc1

mut y = 3
inc1(ref y)
cassert y == 4

comb banner() { puts "hello" }
type T_noarg = comb() -> ()
comb execute_method(fn:T_noarg) {  // explicit type for fn (declared ahead)
  fn() // prints hello when banner passed as argument
}

execute_method(banner)     // OK

In Pyrope, to call a method, parenthesis are needed only when the method has arguments. This is needed to distinguish for higher order functions that need to distinguish between a function call and a pass of the lambda.

Output tuple

Pyrope everything is a tuple, even the output or return from a lambda. The output type always uses parenthesis.

comb ret1() -> (a:int) {
  a = 1
}

comb ret3() -> (a, b) {
  a = 3
  b = 4
}

const a1 = ret1()
cassert a1.a == 1 and a1 == 1  // single-field tuple auto-unwraps

const a3 = ret3()
cassert a3.a == 3 and a3.b == 4

const (x1, x2) = ret3()
cassert x1 == 3 and x2 == 4

Attributes

Variables can have attributes. Attributes can only be integer, bool, or string. Depending on the type, they are initialized to 0, false, or "".

Stateful behavior can be modeled as a tuple with fields and methods. The tuple fields hold the state, and the methods operate on it via ref self.

mut p1 = (
  mut found_once:bool = false,
  mod call(ref self, a) -> (result) {
    self.found_once or= (a == 0)
    result = a + 1
  }
)

mut p2 = p1       // copy
mut p3 = ref p1   // reference

test "testing p1" {
  assert p1.found_once == false
  assert p2.found_once == false

  cassert p1.call(3) == 4
  assert p1.found_once == false

  cassert p1.call(0) == 1
  assert p1.found_once == true

  cassert p1.call(50) == 51
  assert p1.found_once == true
  assert p2.found_once == false
  assert p3.found_once == true
}

mut p1 = (
  mut found_once:bool = false,
  mod setter(ref self, a) {
    self.found_once or= (a == 0)
    self._result = a + 1
  },
  mut _result = 0,
  comb getter(self) { self._result }
)

mut p2 = p1       // copy
mut p3 = ref p1   // reference

test "testing p1" {
  assert p1.found_once == false
  assert p2.found_once == false

  p1 = 3                         // calls setter
  cassert p1 == 4                // calls getter
  assert p1.found_once == false

  p1 = 0
  cassert p1 == 1
  assert p1.found_once == true

  p1 = 50
  cassert p1 == 51
  assert p1.found_once == true
  assert p2.found_once == false
  assert p3.found_once == true
}

Methods

Pyrope arguments are by value, unless the ref keyword is used. ref is needed when a method intends to update the tuple contents. In this case, ref self argument behaves like a pass by reference in non-hardware languages. This means that the tuple fields are updated as the method executes, it does not wait until the method finishes execution. A method without the ref keyword is a pass by value call. Since all the inputs are immutable by default (const), any self updates should generate a compile error.

const Nested_call = (
  mut x = 1,
  comb outter(ref self) { self.x = 100; self.inner(); self.x = 5 },
  comb inner(self) { assert self.x == 100 },
  comb faulty(self) { self.x = 55 }, // error: immutable self
  comb okcall(ref self) { self.x = 55 }
)

self can also be returned but this behaves like a normal copy by value variable return.

mut a_1 = (
  mut x:u10,
  comb f1(ref self, x) -> (self) { // BOTH ref self and return self is OK
    self.x = x
    self
  }
)

a_1.f1(3)
mut a_2 = a_1.f1(4)  // a_2 is updated, not a_1
cassert a_1.x == 3 and a_2.x == 4

// Same behavior as in a function with UFCS
comb set_x(ref self, x) { self.x = x }

a_1.set_x(10)
mut a_3 = a_1.set_x(20)
cassert a_1.x == 10 and a_3.x == 20

Since UFCS does not allow shadowing, a wrapper must be built or a compile error is generated.

mut counter = (
  ,mut val:i32 = 0
  ,comb inc(ref self, v) { self.var += v }
)

assert counter.val == 0
counter.inc(3)
assert counter.val == 3

comb inc(ref self, v) { self.var *= v } // NOT INC but multiply
counter.inc(2)             // error: multiple inc options
assert 44.inc(2) == 8

counter.val = 5
const mul = inc
counter.mul(2)             // call the new mul method with UFCS
assert counter.val == 10

mul(counter, 2)            // also legal
assert counter.val == 20

It is possible to add new methods after the type declaration. In some languages, this is called extension functions.

const t1 = (mut a:u32)

mut x:t1 = (a=3)

comb t1_do_double(ref self) { self.a *= 2 }
t1.double = t1_do_double

mut y:t1 = (a=3)
x.double             // error: double method does not exit
y.double             // OK
assert y.a == 6

Constraining arguments

Arguments can constrain the inputs and input types. Unconstrained input types allow for more freedom and a potentially variable number of arguments generics, but it can be error-prone.

comb foo(self) { puts "comb.foo" }
const a = (
  ,comb foo() {
     comb bar() { puts "bar" }
     puts "mem.foo"
     return (bar=bar)
  }
)
const b = 3
const c = "string"

b.foo         // prints "comb.foo"
b.foo()       // prints "comb.foo"
x = a.foo     // prints "mem.foo"
y = a.foo()   // prints "mem.foo"
x()           // prints "bar"

a.foo.bar()   // prints "mem.foo" and then "bar"
a.foo().bar() // prints "mem.foo" and then "bar"
a.foo().bar   // prints "mem.foo" and then "bar"

c.foo         // prints "comb.foo"

comb foo(self:int) { puts "comb.foo" }
const a = (
  ,comb foo() {
     comb bar() { puts "bar" }
     puts "mem.foo"
     return (bar=bar)
  }
)
const b = 3
const c = "string"

b.foo         // prints "comb.foo"
b.foo()       // prints "comb.foo"
const x = a.foo     // prints "mem.foo"
const y = a.foo()   // prints "mem.foo"
x()           // prints "bar"

a.foo.bar()   // prints "mem.foo" and then "bar"
a.foo().bar() // prints "mem.foo" and then "bar"
a.foo().bar   // prints "mem.foo" and then "bar"

c.foo         // error: undefined 'foo' field/call