Simple shell for evaluating lambda expressions (


  The Lambda Shell

== What is it?

It is a feature-rich shell environment and command-line tool for
evaluating terms of the pure, untyped lambda calculus.  The Lambda
Shell builds on the shell creation framework Shellac, and showcases
most of Shellac's features.

  -- evalutate lambda terms directly from the shell prompt using
     normal or applicative order.  In normal order, one can evaluate
     to normal form, head normal form, or weak head normal form.
  -- define aliases for lambda terms using a top level, non-recursive
     'let' construct.
  -- Show traces of term evaluation, or dump the trace to a file
  -- Count the number of reductions when evaluating terms
  -- test two lambda terms for confluence (that is; if two
     terms, when evaluated to normal form, are alpha equivalant)
  -- programs can be entered from the command line (using the -e option)
     or piped into stdin (using the -s option)
  -- perform Continuation Passing Style (CPS) transforms on terms before
     evaluation using the syntax  '[[ five ]]'

An example session:

$ lambdaShell

The Lambda Shell, version 0.3
Copyright 2005-2006, Robert Dockins

The Lambda Shell comes with ABSOLUTELY NO WARRANTY; for details
type ':nowarranty'.  This is free software, and you are welcome to
redistribute it under certain conditions; type ':gpl'
for details

> (\x y. x) (\a. a) (\b. b)
\a. a
> :load prelude.lam
> :show four
four = succ three
> four
\f x. f (f (f (f x)))
> mul two three
\f x. f (f (f (f (f (f x)))))
> let x = plus six two
> x == eight
> x == nine
not equal
> one
\f x. f x
> [[ one ]]
\f k. k (\x k_0. f x k_0)
> :showcount
showcount on
> sub seven two
\f x. f (f (f (f (f x))))
<<90 reductions>>
> let l = insertSort (cons two (cons three (cons one nil)))
> index zero l
<<463 reductions>>
> index one l
<<2135 reductions>>
> index two l
<<5720 reductions>>
> :quit


== Why do I care?

Because you are a lambda calculus nut, and you just can't get enough.
Or, the lambda shell could be a worthwhile teaching tool.  The command
line features (especially confluence testing) could lend themselves to
automatic grading.  Also, the lambda shell is a good example of
how to write a shell using Shellac.

== How is it licensed?

The Lambda Shell is licensed under the GNU GPL version 2.  See
the LICENSE file for details.

== How do I build it?

The lambda shell uses a Cabal build system.  The follwing
commands assume you have a haskell interpreter in your system
path named 'runhaskell'.  All commands are run from
this directory.  If Shellac is installed as a user package, you
will need to add the '--user' flag to your configure commands.

To install for the whole system:

runhaskell Setup.hs configure
runhaskell Setup.hs build
runhaskell Setup.hs install

To install for a single user:

runhaskell Setup.hs configure --prefix=/home/<username>
runhaskell Setup.hs build
runhaskell Setup.hs install --user

== Who is responsable for this mess?

You can send bug reports, rants and comments to:

  Robert Dockins <robdockins AT>