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.
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
equal
> 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
one
<<463 reductions>>
> index one l
two
<<2135 reductions>>
> index two l
three
<<5720 reductions>>
> :quit
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== 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 fastmail.fm>
