Support fn_capture as lambda.

I think it's safe to make it always a lambda.

Python expressions never cross line boundaries, including in Lambdas.

In general, this means that not all gleam expressions can map to python
expressions (consider case->match, where match is a statement in python,
not an expression). But I *think* it means that any expression that can
map to a python expression can be wrapped in a lambda like this.

The parens make it so the expression is self-contained and you can call
it.

src/generator.gleam

@@ -150,6 +150,18 @@ fn generate_expression(expression: python.Expression) {
       ))
       |> string_builder.append(")")
 
+    python.Lambda(arguments, body) -> {
+      string_builder.from_string("(lambda ")
+      |> string_builder.append_builder(generator_helpers.generate_plural(
+        arguments,
+        generate_expression,
+        ", ",
+      ))
+      |> string_builder.append(": ")
+      |> string_builder.append_builder(generate_expression(body))
+      |> string_builder.append(")")
+    }
+
     python.Call(function, arguments) ->
       string_builder.new()
       |> string_builder.append_builder(generate_expression(function))

src/python.gleam

@@ -41,6 +41,7 @@ pub type Expression {
   Not(Expression)
   Panic(Expression)
   Todo(Expression)
+  Lambda(args: List(Expression), body: Expression)
   List(elements: List(Expression))
   ListWithRest(elements: List(Expression), rest: Expression)
   TupleIndex(tuple: Expression, index: Int)

src/transformer.gleam

@@ -123,6 +123,21 @@ fn transform_expression(expression: glance.Expression) -> python.Expression {
       )
     }
 
+    glance.FnCapture(label, function, arguments_before, arguments_after) -> {
+      let argument_expressions =
+        list.concat([
+          arguments_before,
+          [glance.Field(label, glance.Variable("fn_capture"))],
+          arguments_after,
+        ])
+        |> list.map(transform_call_argument)
+
+      python.Lambda(
+        [python.Variable("fn_capture")],
+        python.Call(transform_expression(function), argument_expressions),
+      )
+    }
+
     glance.Tuple(expressions) ->
       expressions
       |> list.map(transform_expression)
@@ -134,35 +149,12 @@ fn transform_expression(expression: glance.Expression) -> python.Expression {
     glance.FieldAccess(container: expression, label:) ->
       python.FieldAccess(transform_expression(expression), label)
 
-    glance.BinaryOperator(glance.Pipe, left, glance.Variable(function)) -> {
-      // simple pipe left |> foo
-      python.Call(python.Variable(function), [
+    glance.BinaryOperator(glance.Pipe, left, right) -> {
+      python.Call(transform_expression(right), [
         python.UnlabelledField(transform_expression(left)),
       ])
     }
 
-    glance.BinaryOperator(
-      glance.Pipe,
-      left,
-      glance.FnCapture(
-        label,
-        glance.Variable(function),
-        arguments_before,
-        arguments_after,
-      ),
-    ) -> {
-      let argument_expressions =
-        list.concat([
-          arguments_before,
-          [glance.Field(label, left)],
-          arguments_after,
-        ])
-        |> list.map(transform_call_argument)
-      python.Call(python.Variable(function), argument_expressions)
-    }
-    glance.BinaryOperator(glance.Pipe, _, right) -> {
-      panic as "I don't know how to handle this structure of pipe"
-    }
     glance.BinaryOperator(name, left, right) ->
       transform_binop(name, left, right)
 
@@ -187,8 +179,7 @@ fn transform_expression(expression: glance.Expression) -> python.Expression {
     glance.BitString(_) as expr
     | glance.Block(_) as expr
     | glance.Case(_, _) as expr
-    | glance.Fn(_, _, _) as expr
-    | glance.FnCapture(_, _, _, _) as expr -> {
+    | glance.Fn(_, _, _) as expr -> {
       pprint.debug(expr)
       todo as "Several expressions are not implemented yet"
     }

test/expression_test.gleam

@@ -507,7 +507,7 @@ pub fn capture_pipe_test() {
     "from gleam_builtins import *
 
 def main():
-    return println(\"a\", \"foo\", \"b\")",
+    return (lambda fn_capture: println(\"a\", fn_capture, \"b\"))(\"foo\")",
   )
 }
 
@@ -539,6 +539,22 @@ def main():
   )
 }
 
+pub fn fn_capture_test() {
+  "fn main() {
+      let x = foo(\"a\", _, \"b\")
+      x(\"c\")
+  }"
+  |> macabre.compile
+  |> should.be_ok
+  |> should.equal(
+    "from gleam_builtins import *
+
+def main():
+    x = (lambda fn_capture: foo(\"a\", fn_capture, \"b\"))
+    return x(\"c\")",
+  )
+}
+
 pub fn record_update_test() {
   "pub type Foo {
     Bar(a: Int, b: String)