if()'s arguments are evaluated lazily. Fixes #339

6 files changed, 76 insertions, 31 deletions

diff --git a/scss/ast.py b/scss/ast.py
index 122fbf2..16a4518 100644
--- a/scss/ast.py
+++ b/scss/ast.py
@@ -533,3 +533,33 @@ class AlphaFunctionLiteral(Expression):
             # TODO compress
             contents = child.render()
         return Function('opacity=' + contents, 'alpha', quotes=None)
+
+
+class TernaryOp(Expression):
+    """Sass implements this with a function:
+
+        prop: if(condition, true-value, false-value);
+
+    However, the second and third arguments are guaranteed not to be evaluated
+    unless necessary.  Functions always receive evaluated arguments, so this is
+    a syntactic construct in disguise.
+    """
+    def __repr__(self):
+        return '<%s(%r, %r, %r)>' % (
+            self.__class__.__name__,
+            self.condition,
+            self.true_expression,
+            self.false_expression,
+        )
+
+    def __init__(self, list_literal):
+        args = list_literal.items
+        if len(args) != 3:
+            raise SyntaxError("if() must have exactly 3 arguments")
+        self.condition, self.true_expression, self.false_expression = args
+
+    def evaluate(self, calculator, divide=False):
+        if self.condition.evaluate(calculator, divide=True):
+            return self.true_expression.evaluate(calculator, divide=True)
+        else:
+            return self.false_expression.evaluate(calculator, divide=True)
diff --git a/scss/extension/core.py b/scss/extension/core.py
index 966a830..8927c00 100644
--- a/scss/extension/core.py
+++ b/scss/extension/core.py
@@ -941,11 +941,3 @@ def comparable(number1, number2):
 
 
 # TODO call
-
-
-# ------------------------------------------------------------------------------
-# Miscellaneous
-
-@ns.declare
-def if_(condition, if_true, if_false=Null()):
-    return if_true if condition else if_false
diff --git a/scss/grammar/expression.g b/scss/grammar/expression.g
index 6fef9be..2ee1e99 100644
--- a/scss/grammar/expression.g
+++ b/scss/grammar/expression.g
@@ -26,6 +26,7 @@ from scss.ast import MapLiteral
 from scss.ast import ArgspecLiteral
 from scss.ast import FunctionLiteral
 from scss.ast import AlphaFunctionLiteral
+from scss.ast import TernaryOp
 from scss.cssdefs import unescape
 from scss.types import Color
 from scss.types import Function
@@ -100,6 +101,7 @@ parser SassExpression:
     token ALPHA_FUNCTION: "alpha(?=[(])"
     token OPACITY: "((?i)opacity)"
     token URL_FUNCTION: "url(?=[(])"
+    token IF_FUNCTION: "if(?=[(])"
     # This must come AFTER the above
     token FNCT: "[-a-zA-Z_][-a-zA-Z0-9_]*(?=\()"
 
@@ -248,6 +250,9 @@ parser SassExpression:
                 {{ return AlphaFunctionLiteral(atom) }}
             | argspec RPAR          {{ return CallOp("alpha", argspec) }}
             )
+        # This is a ternary operator, disguised as a function
+        | IF_FUNCTION LPAR expr_lst RPAR
+            {{ return TernaryOp(expr_lst) }}
         | LITERAL_FUNCTION LPAR interpolated_function RPAR
             {{ return Interpolation.maybe(interpolated_function, type=Function, function_name=LITERAL_FUNCTION) }}
         | FNCT LPAR argspec RPAR    {{ return CallOp(FNCT, argspec) }}
diff --git a/scss/grammar/expression.py b/scss/grammar/expression.py
index d3e5cbd..8b0dd82 100644
--- a/scss/grammar/expression.py
+++ b/scss/grammar/expression.py
@@ -26,6 +26,7 @@ from scss.ast import MapLiteral
 from scss.ast import ArgspecLiteral
 from scss.ast import FunctionLiteral
 from scss.ast import AlphaFunctionLiteral
+from scss.ast import TernaryOp
 from scss.cssdefs import unescape
 from scss.types import Color
 from scss.types import Function
@@ -87,6 +88,7 @@ class SassExpressionScanner(Scanner):
         ('ALPHA_FUNCTION', 'alpha(?=[(])'),
         ('OPACITY', '((?i)opacity)'),
         ('URL_FUNCTION', 'url(?=[(])'),
+        ('IF_FUNCTION', 'if(?=[(])'),
         ('FNCT', '[-a-zA-Z_][-a-zA-Z0-9_]*(?=\\()'),
         ('BAREWORD', '(?!\\d)(\\\\[0-9a-fA-F]{1,6}|\\\\.|[-a-zA-Z0-9_])+'),
         ('BANG_IMPORTANT', '!\\s*important'),
@@ -349,6 +351,12 @@ class SassExpression(Parser):
                 argspec = self.argspec()
                 RPAR = self._scan('RPAR')
                 return CallOp("alpha", argspec)
+        elif _token_ == 'IF_FUNCTION':
+            IF_FUNCTION = self._scan('IF_FUNCTION')
+            LPAR = self._scan('LPAR')
+            expr_lst = self.expr_lst()
+            RPAR = self._scan('RPAR')
+            return TernaryOp(expr_lst)
         elif _token_ == 'LITERAL_FUNCTION':
             LITERAL_FUNCTION = self._scan('LITERAL_FUNCTION')
             LPAR = self._scan('LPAR')
@@ -537,9 +545,9 @@ class SassExpression(Parser):
         return Interpolation.maybe(parts)
 
     atom_chks_ = frozenset(['BAREWORD', 'INTERP_START'])
-    expr_map_or_list_rsts__ = frozenset(['LPAR', 'DOUBLE_QUOTE', 'VAR', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'INTERP_START', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'NUM', 'RPAR', 'FNCT', 'NOT', 'BANG_IMPORTANT', 'SINGLE_QUOTE', '","'])
-    u_expr_chks = frozenset(['LPAR', 'DOUBLE_QUOTE', 'BAREWORD', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'ALPHA_FUNCTION', 'VAR', 'NUM', 'FNCT', 'LITERAL_FUNCTION', 'BANG_IMPORTANT', 'SINGLE_QUOTE'])
-    m_expr_rsts = frozenset(['LPAR', 'DOUBLE_QUOTE', 'SUB', 'ALPHA_FUNCTION', 'RPAR', 'MUL', 'INTERP_END', 'BANG_IMPORTANT', 'DIV', 'LE', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NE', 'LT', 'NUM', '":"', 'LITERAL_FUNCTION', 'GT', 'END', 'SIGN', 'BAREWORD', 'GE', 'FNCT', 'VAR', 'EQ', 'AND', 'ADD', 'SINGLE_QUOTE', 'NOT', 'MOD', 'OR', '","'])
+    expr_map_or_list_rsts__ = frozenset(['LPAR', 'RPAR', 'BANG_IMPORTANT', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'INTERP_START', 'DOUBLE_QUOTE', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'NUM', 'VAR', 'FNCT', 'NOT', 'IF_FUNCTION', 'SINGLE_QUOTE', '","'])
+    u_expr_chks = frozenset(['LPAR', 'DOUBLE_QUOTE', 'BANG_IMPORTANT', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'INTERP_START', 'VAR', 'NUM', 'FNCT', 'LITERAL_FUNCTION', 'IF_FUNCTION', 'SINGLE_QUOTE'])
+    m_expr_rsts = frozenset(['LPAR', 'DOUBLE_QUOTE', 'SUB', 'ALPHA_FUNCTION', 'RPAR', 'MUL', 'INTERP_END', 'BANG_IMPORTANT', 'DIV', 'LE', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NE', 'LT', 'NUM', '":"', 'LITERAL_FUNCTION', 'IF_FUNCTION', 'GT', 'END', 'SIGN', 'BAREWORD', 'GE', 'FNCT', 'VAR', 'EQ', 'AND', 'ADD', 'SINGLE_QUOTE', 'NOT', 'MOD', 'OR', '","'])
     interpolated_bare_url_rsts_ = frozenset(['RPAR', 'INTERP_START', 'BAREURL', 'SPACE'])
     argspec_items_rsts = frozenset(['RPAR', 'END', '","'])
     expr_slst_chks = frozenset(['INTERP_END', 'RPAR', 'END', '":"', '","'])
@@ -547,39 +555,39 @@ class SassExpression(Parser):
     goal_interpolated_literal_rsts = frozenset(['END', 'INTERP_START'])
     expr_map_or_list_rsts = frozenset(['RPAR', '":"', '","'])
     goal_interpolated_literal_with_vars_rsts = frozenset(['VAR', 'END', 'INTERP_START'])
-    argspec_item_chks = frozenset(['LPAR', 'DOUBLE_QUOTE', 'VAR', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'INTERP_START', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'NUM', 'FNCT', 'NOT', 'BANG_IMPORTANT', 'SINGLE_QUOTE'])
+    argspec_item_chks = frozenset(['LPAR', 'DOUBLE_QUOTE', 'BANG_IMPORTANT', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'INTERP_START', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'NUM', 'VAR', 'FNCT', 'NOT', 'IF_FUNCTION', 'SINGLE_QUOTE'])
     a_expr_chks = frozenset(['ADD', 'SUB'])
     interpolated_function_parens_rsts = frozenset(['LPAR', 'RPAR', 'INTERP_START'])
-    expr_slst_rsts = frozenset(['LPAR', 'DOUBLE_QUOTE', 'VAR', 'END', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'INTERP_START', 'FNCT', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'NUM', 'RPAR', '":"', 'NOT', 'INTERP_END', 'BANG_IMPORTANT', 'SINGLE_QUOTE', '","'])
-    interpolated_bareword_rsts = frozenset(['LPAR', 'DOUBLE_QUOTE', 'SUB', 'ALPHA_FUNCTION', 'RPAR', 'MUL', 'INTERP_END', 'BANG_IMPORTANT', 'DIV', 'LE', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NE', 'LT', 'NUM', '":"', 'LITERAL_FUNCTION', 'GT', 'END', 'SPACE', 'SIGN', 'BAREWORD', 'GE', 'FNCT', 'VAR', 'EQ', 'AND', 'ADD', 'SINGLE_QUOTE', 'NOT', 'MOD', 'OR', '","'])
-    atom_rsts__ = frozenset(['LPAR', 'DOUBLE_QUOTE', 'SUB', 'ALPHA_FUNCTION', 'RPAR', 'VAR', 'MUL', 'INTERP_END', 'BANG_IMPORTANT', 'DIV', 'LE', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NE', 'LT', 'NUM', '":"', 'LITERAL_FUNCTION', 'GT', 'END', 'SIGN', 'BAREWORD', 'GE', 'FNCT', 'UNITS', 'EQ', 'AND', 'ADD', 'SINGLE_QUOTE', 'NOT', 'MOD', 'OR', '","'])
-    or_expr_rsts = frozenset(['LPAR', 'DOUBLE_QUOTE', 'ALPHA_FUNCTION', 'RPAR', 'INTERP_END', 'BANG_IMPORTANT', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NUM', '":"', 'BAREWORD', 'END', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'FNCT', 'VAR', 'OR', 'NOT', 'SINGLE_QUOTE', '","'])
+    expr_slst_rsts = frozenset(['LPAR', 'DOUBLE_QUOTE', 'ALPHA_FUNCTION', 'RPAR', 'INTERP_END', 'BANG_IMPORTANT', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NUM', '":"', 'BAREWORD', 'IF_FUNCTION', 'END', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'FNCT', 'VAR', 'NOT', 'SINGLE_QUOTE', '","'])
+    interpolated_bareword_rsts = frozenset(['LPAR', 'DOUBLE_QUOTE', 'SUB', 'ALPHA_FUNCTION', 'RPAR', 'MUL', 'INTERP_END', 'BANG_IMPORTANT', 'DIV', 'LE', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NE', 'LT', 'NUM', '":"', 'LITERAL_FUNCTION', 'IF_FUNCTION', 'GT', 'END', 'SPACE', 'SIGN', 'BAREWORD', 'GE', 'FNCT', 'VAR', 'EQ', 'AND', 'ADD', 'SINGLE_QUOTE', 'NOT', 'MOD', 'OR', '","'])
+    atom_rsts__ = frozenset(['LPAR', 'DOUBLE_QUOTE', 'SUB', 'ALPHA_FUNCTION', 'RPAR', 'VAR', 'MUL', 'INTERP_END', 'BANG_IMPORTANT', 'DIV', 'LE', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NE', 'LT', 'NUM', '":"', 'LITERAL_FUNCTION', 'IF_FUNCTION', 'GT', 'END', 'SIGN', 'BAREWORD', 'GE', 'FNCT', 'UNITS', 'EQ', 'AND', 'ADD', 'SINGLE_QUOTE', 'NOT', 'MOD', 'OR', '","'])
+    or_expr_rsts = frozenset(['LPAR', 'DOUBLE_QUOTE', 'ALPHA_FUNCTION', 'RPAR', 'INTERP_END', 'BANG_IMPORTANT', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NUM', '":"', 'BAREWORD', 'IF_FUNCTION', 'END', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'FNCT', 'VAR', 'OR', 'NOT', 'SINGLE_QUOTE', '","'])
     argspec_chks_ = frozenset(['END', 'RPAR'])
     interpolated_string_single_rsts = frozenset(['SINGLE_QUOTE', 'INTERP_START'])
-    interpolated_bareword_rsts_ = frozenset(['LPAR', 'DOUBLE_QUOTE', 'SUB', 'ALPHA_FUNCTION', 'RPAR', 'MUL', 'DIV', 'BANG_IMPORTANT', 'INTERP_END', 'LE', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NE', 'LT', 'NUM', '":"', 'BAREWORD', 'GT', 'END', 'SPACE', 'SIGN', 'LITERAL_FUNCTION', 'GE', 'FNCT', 'VAR', 'EQ', 'AND', 'ADD', 'SINGLE_QUOTE', 'NOT', 'MOD', 'OR', '","'])
-    and_expr_rsts = frozenset(['LPAR', 'DOUBLE_QUOTE', 'ALPHA_FUNCTION', 'RPAR', 'INTERP_END', 'BANG_IMPORTANT', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NUM', '":"', 'BAREWORD', 'END', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'FNCT', 'VAR', 'AND', 'OR', 'NOT', 'SINGLE_QUOTE', '","'])
-    comparison_rsts = frozenset(['LPAR', 'DOUBLE_QUOTE', 'ALPHA_FUNCTION', 'RPAR', 'INTERP_END', 'BANG_IMPORTANT', 'LE', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NE', 'LT', 'NUM', '":"', 'LITERAL_FUNCTION', 'GT', 'END', 'SIGN', 'BAREWORD', 'ADD', 'FNCT', 'VAR', 'EQ', 'AND', 'GE', 'SINGLE_QUOTE', 'NOT', 'OR', '","'])
+    interpolated_bareword_rsts_ = frozenset(['LPAR', 'DOUBLE_QUOTE', 'SUB', 'ALPHA_FUNCTION', 'RPAR', 'MUL', 'DIV', 'BANG_IMPORTANT', 'INTERP_END', 'LE', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NE', 'LT', 'NUM', '":"', 'BAREWORD', 'IF_FUNCTION', 'GT', 'END', 'SPACE', 'SIGN', 'LITERAL_FUNCTION', 'GE', 'FNCT', 'VAR', 'EQ', 'AND', 'ADD', 'SINGLE_QUOTE', 'NOT', 'MOD', 'OR', '","'])
+    and_expr_rsts = frozenset(['LPAR', 'DOUBLE_QUOTE', 'ALPHA_FUNCTION', 'RPAR', 'INTERP_END', 'BANG_IMPORTANT', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NUM', '":"', 'BAREWORD', 'IF_FUNCTION', 'END', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'FNCT', 'VAR', 'AND', 'SINGLE_QUOTE', 'NOT', 'OR', '","'])
+    comparison_rsts = frozenset(['LPAR', 'DOUBLE_QUOTE', 'ALPHA_FUNCTION', 'RPAR', 'INTERP_END', 'BANG_IMPORTANT', 'LE', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NE', 'LT', 'NUM', '":"', 'LITERAL_FUNCTION', 'IF_FUNCTION', 'GT', 'END', 'SIGN', 'BAREWORD', 'ADD', 'FNCT', 'VAR', 'EQ', 'AND', 'GE', 'SINGLE_QUOTE', 'NOT', 'OR', '","'])
     argspec_chks = frozenset(['DOTDOTDOT', 'SLURPYVAR'])
-    atom_rsts_ = frozenset(['KWVAR', 'LPAR', 'DOUBLE_QUOTE', 'SLURPYVAR', 'ALPHA_FUNCTION', 'RPAR', 'BANG_IMPORTANT', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NUM', 'BAREWORD', 'END', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'FNCT', 'VAR', 'OPACITY', 'DOTDOTDOT', 'NOT', 'SINGLE_QUOTE'])
+    atom_rsts_ = frozenset(['KWVAR', 'LPAR', 'DOUBLE_QUOTE', 'SLURPYVAR', 'ALPHA_FUNCTION', 'RPAR', 'BANG_IMPORTANT', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NUM', 'BAREWORD', 'IF_FUNCTION', 'END', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'FNCT', 'VAR', 'OPACITY', 'DOTDOTDOT', 'NOT', 'SINGLE_QUOTE'])
     interpolated_string_double_rsts = frozenset(['DOUBLE_QUOTE', 'INTERP_START'])
     atom_chks__ = frozenset(['COLOR', 'VAR'])
     expr_map_or_list_rsts_ = frozenset(['RPAR', '","'])
-    u_expr_rsts = frozenset(['LPAR', 'DOUBLE_QUOTE', 'BAREWORD', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'ALPHA_FUNCTION', 'SIGN', 'VAR', 'ADD', 'NUM', 'FNCT', 'LITERAL_FUNCTION', 'BANG_IMPORTANT', 'SINGLE_QUOTE'])
+    u_expr_rsts = frozenset(['LPAR', 'DOUBLE_QUOTE', 'BANG_IMPORTANT', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'INTERP_START', 'SIGN', 'VAR', 'ADD', 'NUM', 'FNCT', 'LITERAL_FUNCTION', 'IF_FUNCTION', 'SINGLE_QUOTE'])
     interpolated_url_chks = frozenset(['INTERP_START_URL_HACK', 'BAREURL_HEAD_HACK'])
-    atom_chks = frozenset(['KWVAR', 'LPAR', 'DOUBLE_QUOTE', 'BANG_IMPORTANT', 'END', 'SLURPYVAR', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'DOTDOTDOT', 'INTERP_START', 'RPAR', 'LITERAL_FUNCTION', 'ADD', 'NUM', 'VAR', 'FNCT', 'NOT', 'SIGN', 'SINGLE_QUOTE'])
-    interpolated_url_rsts = frozenset(['LPAR', 'DOUBLE_QUOTE', 'VAR', 'SINGLE_QUOTE', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'INTERP_START', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'NUM', 'FNCT', 'NOT', 'INTERP_START_URL_HACK', 'BANG_IMPORTANT', 'BAREURL_HEAD_HACK'])
+    atom_chks = frozenset(['KWVAR', 'LPAR', 'DOUBLE_QUOTE', 'SLURPYVAR', 'ALPHA_FUNCTION', 'RPAR', 'BANG_IMPORTANT', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NUM', 'BAREWORD', 'IF_FUNCTION', 'END', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'FNCT', 'VAR', 'DOTDOTDOT', 'NOT', 'SINGLE_QUOTE'])
+    interpolated_url_rsts = frozenset(['LPAR', 'DOUBLE_QUOTE', 'BANG_IMPORTANT', 'SINGLE_QUOTE', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'INTERP_START', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'NUM', 'VAR', 'FNCT', 'NOT', 'INTERP_START_URL_HACK', 'IF_FUNCTION', 'BAREURL_HEAD_HACK'])
     comparison_chks = frozenset(['GT', 'GE', 'NE', 'LT', 'LE', 'EQ'])
-    argspec_items_rsts_ = frozenset(['KWVAR', 'LPAR', 'DOUBLE_QUOTE', 'VAR', 'END', 'SLURPYVAR', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'DOTDOTDOT', 'INTERP_START', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'NUM', 'RPAR', 'FNCT', 'NOT', 'BANG_IMPORTANT', 'SINGLE_QUOTE'])
-    a_expr_rsts = frozenset(['LPAR', 'DOUBLE_QUOTE', 'SUB', 'ALPHA_FUNCTION', 'RPAR', 'INTERP_END', 'BANG_IMPORTANT', 'LE', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NE', 'LT', 'NUM', '":"', 'LITERAL_FUNCTION', 'GT', 'END', 'SIGN', 'BAREWORD', 'GE', 'FNCT', 'VAR', 'EQ', 'AND', 'ADD', 'SINGLE_QUOTE', 'NOT', 'OR', '","'])
+    argspec_items_rsts_ = frozenset(['KWVAR', 'LPAR', 'DOUBLE_QUOTE', 'SLURPYVAR', 'ALPHA_FUNCTION', 'RPAR', 'BANG_IMPORTANT', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NUM', 'BAREWORD', 'IF_FUNCTION', 'END', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'FNCT', 'VAR', 'DOTDOTDOT', 'NOT', 'SINGLE_QUOTE'])
+    a_expr_rsts = frozenset(['LPAR', 'DOUBLE_QUOTE', 'SUB', 'ALPHA_FUNCTION', 'RPAR', 'INTERP_END', 'BANG_IMPORTANT', 'LE', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NE', 'LT', 'NUM', '":"', 'LITERAL_FUNCTION', 'IF_FUNCTION', 'GT', 'END', 'SIGN', 'BAREWORD', 'GE', 'FNCT', 'VAR', 'EQ', 'AND', 'ADD', 'SINGLE_QUOTE', 'NOT', 'OR', '","'])
     interpolated_string_rsts = frozenset(['DOUBLE_QUOTE', 'SINGLE_QUOTE'])
-    interpolated_bareword_rsts__ = frozenset(['LPAR', 'DOUBLE_QUOTE', 'SUB', 'ALPHA_FUNCTION', 'RPAR', 'MUL', 'INTERP_END', 'BANG_IMPORTANT', 'DIV', 'LE', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NE', 'LT', 'NUM', '":"', 'LITERAL_FUNCTION', 'GT', 'END', 'SIGN', 'BAREWORD', 'GE', 'FNCT', 'VAR', 'EQ', 'AND', 'ADD', 'SINGLE_QUOTE', 'NOT', 'MOD', 'OR', '","'])
+    interpolated_bareword_rsts__ = frozenset(['LPAR', 'DOUBLE_QUOTE', 'SUB', 'ALPHA_FUNCTION', 'RPAR', 'MUL', 'INTERP_END', 'BANG_IMPORTANT', 'DIV', 'LE', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NE', 'LT', 'NUM', '":"', 'LITERAL_FUNCTION', 'IF_FUNCTION', 'GT', 'END', 'SIGN', 'BAREWORD', 'GE', 'FNCT', 'VAR', 'EQ', 'AND', 'ADD', 'SINGLE_QUOTE', 'NOT', 'MOD', 'OR', '","'])
     m_expr_chks = frozenset(['MUL', 'DIV', 'MOD'])
     goal_interpolated_literal_with_vars_rsts_ = frozenset(['VAR', 'INTERP_START'])
     interpolated_bare_url_rsts = frozenset(['RPAR', 'INTERP_START'])
-    argspec_items_chks = frozenset(['KWVAR', 'LPAR', 'DOUBLE_QUOTE', 'VAR', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'INTERP_START', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'NUM', 'FNCT', 'NOT', 'BANG_IMPORTANT', 'SINGLE_QUOTE'])
-    argspec_rsts = frozenset(['KWVAR', 'LPAR', 'DOUBLE_QUOTE', 'BANG_IMPORTANT', 'END', 'SLURPYVAR', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'DOTDOTDOT', 'INTERP_START', 'RPAR', 'LITERAL_FUNCTION', 'ADD', 'NUM', 'VAR', 'FNCT', 'NOT', 'SIGN', 'SINGLE_QUOTE'])
-    atom_rsts = frozenset(['LPAR', 'DOUBLE_QUOTE', 'BANG_IMPORTANT', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'INTERP_START', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'NUM', 'VAR', 'FNCT', 'NOT', 'RPAR', 'SINGLE_QUOTE'])
-    argspec_items_rsts__ = frozenset(['KWVAR', 'LPAR', 'DOUBLE_QUOTE', 'VAR', 'SLURPYVAR', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'DOTDOTDOT', 'INTERP_START', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'NUM', 'FNCT', 'NOT', 'BANG_IMPORTANT', 'SINGLE_QUOTE'])
-    argspec_rsts_ = frozenset(['KWVAR', 'LPAR', 'DOUBLE_QUOTE', 'BANG_IMPORTANT', 'END', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'INTERP_START', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'NUM', 'VAR', 'FNCT', 'NOT', 'RPAR', 'SINGLE_QUOTE'])
+    argspec_items_chks = frozenset(['KWVAR', 'LPAR', 'DOUBLE_QUOTE', 'BANG_IMPORTANT', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'INTERP_START', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'NUM', 'VAR', 'FNCT', 'NOT', 'IF_FUNCTION', 'SINGLE_QUOTE'])
+    argspec_rsts = frozenset(['KWVAR', 'LPAR', 'DOUBLE_QUOTE', 'SLURPYVAR', 'ALPHA_FUNCTION', 'RPAR', 'BANG_IMPORTANT', 'URL_FUNCTION', 'INTERP_START', 'COLOR', 'NUM', 'BAREWORD', 'IF_FUNCTION', 'END', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'FNCT', 'VAR', 'DOTDOTDOT', 'NOT', 'SINGLE_QUOTE'])
+    atom_rsts = frozenset(['BANG_IMPORTANT', 'LPAR', 'DOUBLE_QUOTE', 'IF_FUNCTION', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'INTERP_START', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'NUM', 'VAR', 'FNCT', 'NOT', 'RPAR', 'SINGLE_QUOTE'])
+    argspec_items_rsts__ = frozenset(['KWVAR', 'LPAR', 'DOUBLE_QUOTE', 'BANG_IMPORTANT', 'SLURPYVAR', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'DOTDOTDOT', 'INTERP_START', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'NUM', 'VAR', 'FNCT', 'NOT', 'IF_FUNCTION', 'SINGLE_QUOTE'])
+    argspec_rsts_ = frozenset(['KWVAR', 'LPAR', 'DOUBLE_QUOTE', 'IF_FUNCTION', 'END', 'URL_FUNCTION', 'BAREWORD', 'COLOR', 'ALPHA_FUNCTION', 'INTERP_START', 'BANG_IMPORTANT', 'SIGN', 'LITERAL_FUNCTION', 'ADD', 'NUM', 'VAR', 'FNCT', 'NOT', 'RPAR', 'SINGLE_QUOTE'])
 
 
diff --git a/scss/tests/files/general/if-lazy.css b/scss/tests/files/general/if-lazy.css
new file mode 100644
index 0000000..fb97f11
--- /dev/null
+++ b/scss/tests/files/general/if-lazy.css
@@ -0,0 +1,4 @@
+p {
+  color: red;
+  color: blue;
+}
diff --git a/scss/tests/files/general/if-lazy.scss b/scss/tests/files/general/if-lazy.scss
new file mode 100644
index 0000000..87ee449
--- /dev/null
+++ b/scss/tests/files/general/if-lazy.scss
@@ -0,0 +1,6 @@
+p {
+    // if()'s arguments are evaluated lazily.  $bogus doesn't exist, so if
+    // those expressions are ever evaluated, pyscss will die with an error
+    color: if(false, $bogus, red);
+    color: if(true, blue, $bogus);
+}