Basic case statements working

README.md

@@ -48,8 +48,10 @@ PRs are welcome.
 ### Some of the things I know are missing
 
 - no destructuring/pattern matching in let
+  - some constructs can map straight to python destructuring
+  - others may need a custom case
 - no let assert
-- no case expressions
+- empty tuples are probably broken
 - label aliases aren't supported yet (ie `fn foo(bar bas: Str)`)
 - const definitions aren't supported yet (module.constants)
 - type aliases aren't supported yet (module.type_aliases)

src/compiler/generator.gleam

@@ -4,6 +4,7 @@ import gleam/int
 import gleam/list
 import gleam/option
 import gleam/string_builder.{type StringBuilder}
+import pprint
 import python_prelude
 
 fn generate_import(import_: python.Import) -> StringBuilder {
@@ -191,17 +192,64 @@ fn generate_statement(statement: python.Statement) -> StringBuilder {
       |> string_builder.append(" = ")
       |> string_builder.append_builder(generate_expression(value))
     }
+    python.Match(cases) ->
+      string_builder.new()
+      |> string_builder.append("match _case_subject:\n")
+      |> string_builder.append_builder(
+        generate_cases(cases) |> internal.indent(4),
+      )
+    // TODO: Deal with cases
     python.FunctionDef(function) -> generate_function(function)
   }
 }
 
+fn generate_cases(cases: List(python.MatchCase)) -> StringBuilder {
+  case cases {
+    [] -> string_builder.from_string("pass")
+    cases -> internal.generate_plural(cases, generate_case, "\n")
+  }
+}
+
+fn generate_case(case_: python.MatchCase) -> StringBuilder {
+  string_builder.from_string("case ")
+  |> string_builder.append_builder(generate_pattern(case_.pattern))
+  |> string_builder.append(":\n")
+  |> string_builder.append_builder(
+    generate_block(case_.body) |> internal.indent(4),
+  )
+}
+
+fn generate_pattern(pattern: python.Pattern) -> StringBuilder {
+  case pattern {
+    python.PatternWildcard -> string_builder.from_string("_")
+    python.PatternInt(str)
+    | python.PatternFloat(str)
+    | python.PatternVariable(str) -> string_builder.from_string(str)
+    python.PatternString(str) -> string_builder.from_strings(["\"", str, "\""])
+    python.PatternAssignment(pattern, name) ->
+      generate_pattern(pattern)
+      |> string_builder.append(" as ")
+      |> string_builder.append(name)
+    python.PatternTuple(patterns) ->
+      patterns
+      |> list.map(generate_pattern)
+      |> string_builder.join(", ")
+      |> string_builder.prepend("(")
+      |> string_builder.append(")")
+    python.PatternAlternate(patterns) ->
+      patterns
+      |> list.map(generate_pattern)
+      |> string_builder.join(" | ")
+  }
+}
+
 fn generate_parameter(param: python.FunctionParameter) -> StringBuilder {
   case param {
     python.NameParam(name) -> string_builder.from_string(name)
   }
 }
 
-fn generate_function_body(statements: List(python.Statement)) -> StringBuilder {
+fn generate_block(statements: List(python.Statement)) -> StringBuilder {
   case statements {
     [] -> string_builder.from_string("pass")
     multiple_lines ->
@@ -222,7 +270,7 @@ fn generate_function(function: python.Function) -> StringBuilder {
   ))
   |> string_builder.append("):\n")
   |> string_builder.append_builder(
-    generate_function_body(function.body) |> internal.indent(4),
+    generate_block(function.body) |> internal.indent(4),
   )
 }
 

src/compiler/internal/transformer.gleam

@@ -11,7 +11,11 @@ pub type TransformError {
 }
 
 pub type TransformerContext {
-  TransformerContext(next_function_id: Int, next_block_id: Int)
+  TransformerContext(
+    next_function_id: Int,
+    next_block_id: Int,
+    next_case_id: Int,
+  )
 }
 
 pub type ExpressionReturn {

src/compiler/internal/transformer/patterns.gleam

@@ -0,0 +1,55 @@
+import compiler/python
+import glance
+import gleam/list
+import pprint
+
+// alternative patterns are sent to us a a list of list of patters.
+// the outer list represents alternatives, so 1 | 2 -> becomes [[1], [2]]
+// inner loop represents groupings (see `transform_grouped_pattrns`)
+// so 1, 2 | 3, 5 becomes [1, 2], [3, 5]
+pub fn transform_alternative_patterns(
+  patterns: List(List(glance.Pattern)),
+) -> python.Pattern {
+  case patterns {
+    [] -> panic as "missing pattern"
+    [one_alternative] -> transform_grouped_patterns(one_alternative)
+    multiple_alternatives ->
+      python.PatternAlternate(list.map(
+        multiple_alternatives,
+        transform_grouped_patterns,
+      ))
+  }
+}
+
+// gleam distinguishes between groups of patterns (e.g: case 1, 2 {x, y -> ...})
+// and glance sends those to us as a list of patterns. The python pattern
+// for a group of patterns will always be a single tuple pattern.
+fn transform_grouped_patterns(patterns: List(glance.Pattern)) -> python.Pattern {
+  case patterns {
+    [] -> panic as "missing pattern"
+    [one_item] -> transform_pattern(one_item)
+    multiple_items -> transform_pattern(glance.PatternTuple(multiple_items))
+  }
+}
+
+fn transform_pattern(pattern: glance.Pattern) -> python.Pattern {
+  case pattern {
+    glance.PatternInt(str) -> python.PatternInt(str)
+    glance.PatternFloat(str) -> python.PatternFloat(str)
+    glance.PatternString(str) -> python.PatternString(str)
+    glance.PatternVariable(str) -> python.PatternVariable(str)
+    glance.PatternDiscard("") -> python.PatternWildcard
+    glance.PatternDiscard(str) -> python.PatternVariable("_" <> str)
+    glance.PatternTuple(patterns) ->
+      python.PatternTuple(list.map(patterns, transform_pattern))
+    glance.PatternList(_, _) -> todo as "list patterns are not supported yet"
+    glance.PatternAssignment(pattern, name) ->
+      python.PatternAssignment(transform_pattern(pattern), name)
+    glance.PatternConcatenate(_, _) ->
+      todo as "concatenate patterns are not supported yet"
+    glance.PatternBitString(..) ->
+      todo as "bitstring patterns are not supported yet"
+    glance.PatternConstructor(..) ->
+      todo as "record constructor patterns are not supported yet"
+  }
+}

src/compiler/internal/transformer/statements.gleam

@@ -1,4 +1,5 @@
 import compiler/internal/transformer as internal
+import compiler/internal/transformer/patterns
 import compiler/python
 import glance
 import gleam/int
@@ -23,6 +24,7 @@ pub fn transform_statement_block(
         context: internal.TransformerContext(
           next_function_id: 0,
           next_block_id: 0,
+          next_case_id: 0,
         ),
         statements: [],
       ),
@@ -145,7 +147,7 @@ fn transform_expression(
 
     glance.Block(statements) -> transform_block(context, statements)
 
-    glance.Case(..) -> todo as "case expressions not supported yet"
+    glance.Case(subjects, clauses) -> transform_case(context, subjects, clauses)
 
     glance.TupleIndex(tuple, index) -> {
       transform_expression(context, tuple)
@@ -340,7 +342,7 @@ fn transform_fn(
 fn transform_block(
   context: internal.TransformerContext,
   body: List(glance.Statement),
-) {
+) -> internal.ExpressionReturn {
   let function_name = "_fn_block_" <> int.to_string(context.next_block_id)
   let function =
     python.Function(function_name, [], transform_statement_block(body))
@@ -354,6 +356,70 @@ fn transform_block(
   )
 }
 
+fn transform_case(
+  context: internal.TransformerContext,
+  subjects: List(glance.Expression),
+  clauses: List(glance.Clause),
+) -> internal.ExpressionReturn {
+  let subjects_result = case subjects {
+    [] -> panic("No subjects!")
+    [subject] -> transform_expression(context, subject)
+    multiple -> transform_tuple(context, multiple)
+  }
+  let clause_result =
+    list.fold(
+      clauses,
+      internal.TransformState(subjects_result.context, [], []),
+      fold_case_clause,
+    )
+
+  let function_name = "_fn_case_" <> int.to_string(context.next_case_id)
+  let function =
+    python.Function(function_name, [python.NameParam("_case_subject")], [
+      python.Match(clause_result.item |> list.reverse),
+    ])
+
+  internal.ExpressionReturn(
+    context: internal.TransformerContext(
+      ..subjects_result.context,
+      next_case_id: context.next_case_id + 1,
+    ),
+    statements: list.append(subjects_result.statements, [
+      python.FunctionDef(function),
+    ]),
+    expression: python.Call(python.Variable(function_name), [
+      python.UnlabelledField(subjects_result.expression),
+    ]),
+  )
+}
+
+fn fold_case_clause(
+  state: internal.TransformState(internal.ReversedList(python.MatchCase)),
+  clause: glance.Clause,
+) -> internal.TransformState(internal.ReversedList(python.MatchCase)) {
+  case clause {
+    glance.Clause(guard: option.Some(_), ..) ->
+      todo as "Case guards not implemented yet"
+
+    glance.Clause([pattern_list], option.None, glance.Block(_statements)) -> {
+      todo as "block case clauses not supported yet"
+    }
+
+    glance.Clause(pattern_list, option.None, body) -> {
+      let python_pattern = patterns.transform_alternative_patterns(pattern_list)
+      let body_result = transform_expression(state.context, body)
+
+      internal.merge_state_prepend(state, body_result, fn(expr) {
+        python.MatchCase(python_pattern, [python.Return(expr)])
+      })
+    }
+
+    glance.Clause(..) -> {
+      todo as "multiple clause not implemented yet"
+    }
+  }
+}
+
 fn transform_pipe(
   context: internal.TransformerContext,
   left: glance.Expression,

src/compiler/python.gleam

@@ -55,6 +55,7 @@ pub type Statement {
   Expression(Expression)
   Return(Expression)
   FunctionDef(Function)
+  Match(cases: List(MatchCase))
   SimpleAssignment(name: String, value: Expression)
 }
 
@@ -85,6 +86,23 @@ pub type CustomType {
   CustomType(name: String, parameters: List(String), variants: List(Variant))
 }
 
+pub type MatchCase {
+  // Inner List is collecting tuples together, outer list is patterns that get or'd together
+  // e.g. 1,2 | 3, 4 becomes [[1,2], [3, 4]]
+  MatchCase(pattern: Pattern, body: List(Statement))
+}
+
+pub type Pattern {
+  PatternWildcard
+  PatternInt(value: String)
+  PatternFloat(value: String)
+  PatternString(value: String)
+  PatternVariable(value: String)
+  PatternAssignment(pattern: Pattern, name: String)
+  PatternTuple(value: List(Pattern))
+  PatternAlternate(patterns: List(Pattern))
+}
+
 pub type Function {
   Function(
     name: String,