Support numeric bitstrings

README.md

@@ -61,7 +61,7 @@ Some tasks below are marked easy if you want to get started.
 This is a list of all outstanding `todo` expressions (Gleam todo expressions
 are ) in the codebase, as of the last time that I updated this list.
 
-- imports with attributes are not implemented yet
+- imports with attributes are not handled yet
 - type imports are not implemented yet
 - Unlabelled fields in custom types are not generated yet
   - Given that labelled and unlabelled fields can be mixed on one class, I have

src/compiler/internal/generator/expressions.gleam

@@ -107,6 +107,8 @@ pub fn generate_expression(expression: python.Expression) -> StringBuilder {
 
     python.BinaryOperator(name, left, right) ->
       generate_binop(name, left, right)
+
+    python.BitString(segments) -> generate_bitstring(segments)
   }
 }
 
@@ -161,3 +163,46 @@ fn generate_binop(
   |> string_builder.append(op_string)
   |> string_builder.append_builder(generate_expression(right))
 }
+
+fn generate_bitstring(segments: List(python.BitStringSegment)) -> StringBuilder {
+  string_builder.from_string("gleam_bitstring_segments_to_bytes(")
+  |> string_builder.append_builder(internal.generate_plural(
+    segments,
+    generate_bitstring_segment,
+    ", ",
+  ))
+  |> string_builder.append(")")
+}
+
+fn generate_bitstring_segment(segment: python.BitStringSegment) -> StringBuilder {
+  generate_expression(segment.value)
+  |> string_builder.prepend("(")
+  |> string_builder.append(", [")
+  |> string_builder.append_builder(internal.generate_plural(
+    segment.options,
+    generate_bitstring_segment_option,
+    ", ",
+  ))
+  |> string_builder.append("])")
+}
+
+fn generate_bitstring_segment_option(
+  option: python.BitStringSegmentOption,
+) -> StringBuilder {
+  case option {
+    python.SizeValueOption(expression) ->
+      generate_expression(expression)
+      |> string_builder.prepend("\"SizeValue\", ")
+    python.UnitOption(integer) ->
+      integer
+      |> int.to_string
+      |> string_builder.from_string
+      |> string_builder.prepend("\"Unit\", ")
+    python.FloatOption -> string_builder.from_string("\"Float\", None")
+    python.BigOption -> string_builder.from_string("\"Big\", None")
+    python.LittleOption -> string_builder.from_string("\"Little\", None")
+    python.NativeOption -> string_builder.from_string("\"Native\", None")
+  }
+  |> string_builder.prepend("(")
+  |> string_builder.append(")")
+}

src/compiler/internal/transformer.gleam

@@ -98,14 +98,21 @@ pub fn merge_state_prepend(
   prev: TransformState(ReversedList(a)),
   current: ExpressionReturn,
   map_next: fn(python.Expression) -> a,
-) -> TransformState(List(a)) {
+) -> TransformState(ReversedList(a)) {
   merge_state(
     prev,
     current,
     list.prepend(prev.item, map_next(current.expression)),
   )
 }
 
+pub fn map_state_prepend(
+  prev: TransformState(ReversedList(a)),
+  next: a,
+) -> TransformState(ReversedList(a)) {
+  TransformState(prev.context, prev.statements, prev.item |> list.prepend(next))
+}
+
 pub fn reverse_state_to_return(
   state: TransformState(ReversedList(a)),
   mapper: fn(List(a)) -> python.Expression,

src/compiler/internal/transformer/statements.gleam

@@ -179,9 +179,13 @@ fn transform_expression(
     glance.RecordUpdate(record:, fields:, ..) ->
       transform_record_update(context, record, fields)
 
-    glance.BitString(_) as expr -> {
-      pprint.debug(expr)
-      todo as "BitString expressions not supported yet"
+    glance.BitString(segments) -> {
+      segments
+      |> list.fold(
+        internal.TransformState(context, [], []),
+        fold_bitstring_segment,
+      )
+      |> internal.reverse_state_to_return(python.BitString)
     }
   }
 }
@@ -488,7 +492,7 @@ fn transform_record_update(
   let record_result = transform_expression(context, record)
   fields
   |> list.fold(
-    internal.TransformState(record_result.context, [], []),
+    internal.TransformState(record_result.context, record_result.statements, []),
     fn(state, tuple) {
       internal.merge_state_prepend(
         state,
@@ -502,3 +506,76 @@ fn transform_record_update(
     fields: _,
   ))
 }
+
+fn fold_bitstring_segment(
+  state: internal.TransformState(internal.ReversedList(python.BitStringSegment)),
+  segment: #(
+    glance.Expression,
+    List(glance.BitStringSegmentOption(glance.Expression)),
+  ),
+) -> internal.TransformState(internal.ReversedList(python.BitStringSegment)) {
+  let #(expression, options) = segment
+  let expression_result = transform_expression(state.context, expression)
+  let options_result =
+    options
+    |> list.fold(
+      internal.TransformState(
+        expression_result.context,
+        expression_result.statements,
+        [],
+      ),
+      fold_bitsting_segment_option,
+    )
+
+  internal.TransformState(
+    options_result.context,
+    options_result.statements,
+    list.prepend(
+      state.item,
+      python.BitStringSegment(
+        expression_result.expression,
+        options_result.item |> list.reverse,
+      ),
+    ),
+  )
+}
+
+fn fold_bitsting_segment_option(
+  state: internal.TransformState(
+    internal.ReversedList(python.BitStringSegmentOption),
+  ),
+  option: glance.BitStringSegmentOption(glance.Expression),
+) -> internal.TransformState(
+  internal.ReversedList(python.BitStringSegmentOption),
+) {
+  case option {
+    glance.FloatOption -> internal.map_state_prepend(state, python.FloatOption)
+    glance.LittleOption ->
+      internal.map_state_prepend(state, python.LittleOption)
+    glance.BigOption -> internal.map_state_prepend(state, python.BigOption)
+    glance.NativeOption ->
+      internal.map_state_prepend(state, python.NativeOption)
+    glance.UnitOption(size) ->
+      internal.map_state_prepend(state, python.UnitOption(size))
+    glance.SizeOption(size) ->
+      internal.map_state_prepend(
+        state,
+        python.SizeValueOption(python.Number(size |> int.to_string)),
+      )
+    glance.SizeValueOption(expression) -> {
+      let expression_result = transform_expression(state.context, expression)
+      internal.merge_state_prepend(
+        state,
+        expression_result,
+        python.SizeValueOption,
+      )
+    }
+    glance.SignedOption | glance.UnsignedOption -> {
+      panic as "Signed and unsigned are not valid when constructing bitstrings"
+    }
+    _ -> {
+      pprint.debug(option)
+      todo as "Some bitstring segment options not supported yet"
+    }
+  }
+}

src/compiler/python.gleam

@@ -48,6 +48,20 @@ pub type Expression {
   Call(function: Expression, arguments: List(Field(Expression)))
   RecordUpdate(record: Expression, fields: List(Field(Expression)))
   BinaryOperator(name: BinaryOperator, left: Expression, right: Expression)
+  BitString(List(BitStringSegment))
+}
+
+pub type BitStringSegment {
+  BitStringSegment(value: Expression, options: List(BitStringSegmentOption))
+}
+
+pub type BitStringSegmentOption {
+  SizeValueOption(Expression)
+  UnitOption(Int)
+  FloatOption
+  LittleOption
+  BigOption
+  NativeOption
 }
 
 pub type Statement {

src/python_prelude.gleam

@@ -1,12 +1,14 @@
 pub const gleam_builtins = "
 import dataclasses
+import sys
 import typing
+import struct
 
 class GleamPanic(BaseException):
     pass
 
 
-GleamListElem = typing.TypeVar(\"GleamListElem\")
+GleamListElem = typing.TypeVar('GleamListElem')
 
 
 class GleamList(typing.Generic[GleamListElem]):
@@ -18,11 +20,11 @@ class GleamList(typing.Generic[GleamListElem]):
             strs.append(head.value)
             head = head.tail
 
-        return  \"GleamList([\" + \", \".join(strs) + \"])\"
+        return  'GleamList([' + ', '.join(strs) + '])'
 
 
 class NonEmptyGleamList(GleamList[GleamListElem]):
-    __slots__ = [\"value\", \"tail\"]
+    __slots__ = ['value', 'tail']
     is_empty = False
 
     def __init__(self, value: GleamListElem, tail: GleamList[GleamListElem]):
@@ -41,6 +43,83 @@ def to_gleam_list(elements: list[GleamListElem], tail: GleamList = EmptyGleamLis
     for element in reversed(elements):
         head = NonEmptyGleamList(element, head)
     return head
+
+def gleam_bitstring_segments_to_bytes(*segments):
+    result = bytearray()
+    for segment in segments:
+        result.extend(gleam_bitstring_segment_to_bytes(segment))
+    return bytes(result)
+    
+
+def gleam_bitstring_segment_to_bytes(segment) -> bytes:
+    value, options = segment
+
+    size = None
+    unit = None
+    type = None
+    endianness = 'big'
+    for option in options:
+        match option:
+            case ('SizeValue', size):
+                size = size
+            case('Unit', unit):
+                unit = unit
+            case('Little', None):
+                endianness = 'little'
+            case('Big', None):
+                endianness = 'big'
+            case('Native', None):
+                endianness = sys.byteorder
+            case('Float', None):
+                type = 'float'
+            case _:
+                raise Exception(f'Unexpected bitstring option {option}')
+
+    # Defaults from https://www.erlang.org/doc/system/bit_syntax.html
+    if type == None:
+        type = 'int'
+
+    if size == None:
+        match type:
+            case 'int':
+                size = 8
+            case 'float':
+                size = 64
+
+    if unit == None:
+        match type:
+            case 'int':
+                unit = 1
+            case 'float':
+                unit = 1
+
+    bitsize = unit * size
+    if bitsize % 8:
+        raise Exception(f'Python bitstrings must be byte aligned, but got {bitsize}')
+
+    bytesize = bitsize // 8
+
+    match type:
+        case 'int':
+            return value.to_bytes(bitsize // 8, endianness, signed=value < 0)
+        case 'float':
+            match endianness: 
+                case 'big':
+                    order = '>'
+                case 'little':
+                    order = '<'
+                case 'native':
+                    onder = '='
+            match bitsize:
+                case 32:
+                    fmt = 'f'
+                case  64:
+                    fmt = 'd'
+                case _:
+                    raise Exception('bitstring floats must be 32 or 64 bits')
+            return struct.pack(f'{order}{fmt}', value)
+
+    raise Exception('Unexpected bitstring encountered')
 "
 
 pub const prelude = "from gleam_builtins import *\n\n"