From 4e2a28c9c67244cbdd9aa3ab096ce7dc1a5abb09 Mon Sep 17 00:00:00 2001
From: Juha Jeronen <juha.jeronen@jyu.fi>
Date: Sat, 6 Oct 2018 00:33:00 +0300
Subject: [PATCH] =?UTF-8?q?tentative:=20allow=20default=20values=20for=20a?=
 =?UTF-8?q?rguments=20in=20=CE=BB?=
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---
 macro_extras/main.py |  6 ++++++
 unpythonic/syntax.py | 18 +++++++++++++-----
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/macro_extras/main.py b/macro_extras/main.py
index fb5f6e34..007598d5 100644
--- a/macro_extras/main.py
+++ b/macro_extras/main.py
@@ -264,6 +264,12 @@ def g(*args):
     z = echo("hi there")
     assert z == "hi there"
 
+    echo = λ(myarg="hello")[print(myarg), myarg]
+    result = echo()
+    assert result == "hello"
+    result = echo(3)
+    assert result == 3
+
     myadd = λ(x, y)[print("myadding", x, y), x + y]
     assert myadd(2, 3) == 5
 
diff --git a/unpythonic/syntax.py b/unpythonic/syntax.py
index 079b96db..f90aac39 100644
--- a/unpythonic/syntax.py
+++ b/unpythonic/syntax.py
@@ -526,9 +526,8 @@ def _monadify(value, unpack=True):
 
 # -----------------------------------------------------------------------------
 
-# TODO: support default values for arguments. Requires support in MacroPy for named arguments.
 @macros.expr
-def λ(tree, args, **kw):
+def λ(tree, args, kwargs, **kw):
     """[syntax, expr] Rackety lambda with implicit begin.
 
     (Actually, implicit ``do``, because that gives an internal definition
@@ -543,12 +542,21 @@ def λ(tree, args, **kw):
     Limitations:
 
       - No *args or **kwargs.
-      - No default values for arguments.
     """
-    names = [k.id for k in args]
+    invalids = [x for x in args if type(x) is not Name]
+    if invalids:
+        assert False, "arguments to λ must be name or name=default_value"
+    # To make defaults possible, we abuse the fact that args with defaults
+    # are always declared after args with no defaults. There is fortunately
+    # a similar rule that in a call, arguments passed by name come after
+    # arguments passed by position. Combine these and OrderedDict, and bingo.
+    names = [k.id for k in args] + [k for k in kwargs]
+    if len(set(names)) < len(names):  # may happen if both bare and with-default.
+        assert False, "argument names must be unique in the same λ"
     newtree = do.transform(tree)
     lam = q[lambda: ast_literal[newtree]]
-    lam.args.args = [arg(arg=x) for x in names]  # inject args
+    lam.args.args = [arg(arg=x) for x in names]
+    lam.args.defaults = [kwargs[k] for k in kwargs]  # for the last n args
     return lam
 
 # -----------------------------------------------------------------------------