From 4e987026148fe65c323afbc93cd560c07bf06b3f Mon Sep 17 00:00:00 2001
From: Yale AI Dept <ai@nebula.cs.yale.edu>
Date: Wed, 14 Jul 1993 13:08:00 -0500
Subject: Import to github.

---
 progs/demo/prolog/Interact.hs | 76 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 76 insertions(+)
 create mode 100644 progs/demo/prolog/Interact.hs

(limited to 'progs/demo/prolog/Interact.hs')

diff --git a/progs/demo/prolog/Interact.hs b/progs/demo/prolog/Interact.hs
new file mode 100644
index 0000000..c8bf516
--- /dev/null
+++ b/progs/demo/prolog/Interact.hs
@@ -0,0 +1,76 @@
+--
+-- Interactive utility functions
+-- Mark P. Jones November 1990
+--
+-- uses Haskell B. version 0.99.3
+--
+module Interact(Interactive(..), skip, end, readln, writeln, readch) where
+
+-- The functions defined in this module provide basic facilities for
+-- writing line-oriented interactive programs (i.e. a function mapping
+-- an input string to an appropriate output string).  These definitions
+-- are an enhancement of thos in B+W 7.8
+--
+-- skip p         is an interactive program which consumes no input, produces
+--                no output and then behaves like the interactive program p.
+-- end            is an interactive program which ignores the input and
+--                produces no output.
+-- writeln txt p  is an interactive program which outputs the message txt
+--                and then behaves like the interactive program p
+-- readch act def is an interactive program which reads the first character c
+--                from the input stream and behaves like the interactive
+--                program act c.  If the input character stream is empty,
+--                readch act def prints the default string def and terminates.
+-- 
+-- readln p g     is an interactive program which prints the prompt p and
+--                reads a line (upto the first carriage return, or end of
+--                input) from the input stream.  It then behaves like g line.
+--                Backspace characters included in the input stream are
+--                interpretted in the usual way.
+
+type Interactive = String -> String
+
+--- Interactive program combining forms:
+
+skip                 :: Interactive -> Interactive
+skip p inn             = p inn    -- a dressed up identity function
+
+end                  :: Interactive
+end inn                = ""
+
+writeln              :: String -> Interactive -> Interactive
+writeln txt p inn      = txt ++ p inn
+
+readch               :: (Char -> Interactive) -> String -> Interactive
+readch act def ""     = def
+readch act def (c:cs) = act c cs
+
+readln               :: String -> (String -> Interactive) -> Interactive
+readln prompt g inn    = prompt ++ lineOut 0 line ++ "\n"
+                               ++ g (noBackSpaces line) input'
+                        where line     = before '\n' inn
+                              input'   = after  '\n' inn
+                              after x  = tail . dropWhile (x/=)
+                              before x = takeWhile (x/=)
+
+--- Filter out backspaces etc:
+
+rubout  :: Char -> Bool
+rubout c = (c=='\DEL' || c=='\BS')
+
+lineOut                      :: Int -> String -> String
+lineOut n ""                  = ""
+lineOut n (c:cs)
+          | n>0  && rubout c  = "\BS \BS" ++ lineOut (n-1) cs
+          | n==0 && rubout c  = lineOut 0 cs
+          | otherwise         = c:lineOut (n+1) cs
+
+noBackSpaces :: String -> String
+noBackSpaces  = reverse . delete 0 . reverse
+                where delete n ""          = ""
+                      delete n (c:cs)
+                               | rubout c  = delete (n+1) cs
+                               | n>0       = delete (n-1) cs
+                               | otherwise = c:delete 0 cs
+
+--- End of Interact.hs
-- 
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