X-Git-Url: https://thelambdalab.xyz/gitweb/index.cgi?a=blobdiff_plain;f=scheme.4th;h=1f0211464f792819dcdc204d71295e33ce4527c3;hb=4f600fd1014c0684aad06ec8fdbc8d3ee9c324a4;hp=8581893525ce82400121347baa0aa7eecd7ad9d6;hpb=fb1e77539ea2eea41e87cb50743a2e4df8e8dac5;p=scheme.forth.jl.git

diff --git a/scheme.4th b/scheme.4th
index 8581893..1f02114 100644
--- a/scheme.4th
+++ b/scheme.4th
@@ -82,6 +82,20 @@ variable nextfree
 : objeq? ( obj obj -- bool )
     rot = -rot = and ;
 
+: 2rot ( a1 a2 b1 b2 c1 c2 -- b1 b2 c1 c2 a1 a2 )
+    >R >R ( a1 a2 b1 b2 )
+    2swap ( b1 b2 a1 a2 )
+    R> R> ( b1 b2 a1 a2 c1 c2 )
+    2swap
+;
+
+: -2rot ( a1 a2 b1 b2 c1 c2 -- c1 c2 a1 a2 b1 b2 )
+    2swap ( a1 a2 c1 c2 b1 b2 )
+    >R >R ( a1 a2 c1 c2 )
+    2swap ( c1 c2 a1 a2 )
+    R> R>
+;
+
 \ }}}
 
 \ ---- Pre-defined symbols ---- {{{
@@ -121,6 +135,7 @@ create-symbol quote     quote-symbol
 create-symbol define    define-symbol
 create-symbol set!      set!-symbol
 create-symbol ok        ok-symbol
+create-symbol if        if-symbol
 
 \ }}}
 
@@ -669,6 +684,8 @@ defer read
 
 \ ---- Eval ---- {{{
 
+defer eval
+
 : self-evaluating? ( obj -- obj bool )
     boolean-type istype? if true exit then
     number-type istype? if true exit then
@@ -714,8 +731,6 @@ defer read
 : assignment-val ( obj -- val )
     cdr cdr car ;
 
-defer eval
-
 : eval-definition ( obj env -- res )
     2swap 
     2over 2over ( env obj env obj )
@@ -724,7 +739,7 @@ defer eval
     
     2swap definition-var 2swap ( env var val )
 
-    >R >R 2swap R> R> 2swap ( var val env )
+    2rot ( var val env )
     define-var
 
     ok-symbol
@@ -738,12 +753,40 @@ defer eval
     
     2swap assignment-var 2swap ( env var val )
 
-    >R >R 2swap R> R> 2swap ( var val env )
+    2rot ( var val env )
     set-var
 
     ok-symbol
 ;
 
+: if? ( obj -- obj bool )
+    if-symbol tagged-list? ;
+
+: if-predicate ( ifobj -- pred )
+    cdr car ;
+
+: if-consequent ( ifobj -- conseq )
+    cdr cdr car ;
+
+: if-alternative ( ifobj -- alt|false )
+    cdr cdr cdr
+    2dup nil objeq? if
+        2drop false
+    else
+        car
+    then ;
+
+: false? ( boolobj -- boolean )
+    boolean-type istype? if
+        false boolean-type objeq?
+    else
+        2drop false
+    then
+;
+
+: true? ( boolobj -- boolean )
+    false? invert ;
+
 :noname ( obj env -- result )
     2swap
 
@@ -773,6 +816,20 @@ defer eval
         exit
     then
 
+    if? if
+        2over 2over
+        if-predicate
+        2swap eval 
+
+        true? if
+            if-consequent
+        else
+            if-alternative
+        then
+
+        2swap ['] eval goto
+    then
+
     bold fg red ." Error evaluating expression - unrecognized type. Aborting." reset-term cr
     abort
 ; is eval