Copyright 2014-2022 Mikael Pettersson

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

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ErlScheme

ErlScheme is an implementation of the Scheme programming language for the Erlang/OTP virtual machine.

ErlScheme aims for Scheme R7RS compatibility, with extensions for Erlang interoperability. ErlScheme added features include:

• Calls to Erlang code.

  • (M:F A1 ... An) calls function F of arity n exported from module M

  • (lambda M:F/A) evaluates to function F of arity A exported from module M

• Separately-compiled modules. A module like

    (module meaning)
    (export (/ life 0)) ; or (export life/0)
    (define (life) 42)
  

  in a file "meaning.scm" can be compiled to "meaning.beam", and then called from ErlScheme as (meaning:life) or from Erlang as meaning:life().

• Erlang-like exception handling:

    (try Expr
      (of Var Body)
      (catch EVar Handler)
      (after After))
  

  ErlScheme exceptions have termination semantics, not resumption semantics as specified by R7RS.

• Erlang-like case expressions and pattern matching:

    (case Expr
      ('x 'got_an_x) ; quote symbols to treat them as literals
      (y (when Guard) Body..) ; unquoted symbols are variables
      (_ Default))
  

  Patterns are datums where symbols denote variables if unquoted, and literals when quoted. References to bound variables are equality constraints, as in Erlang.

  Scheme's original (case ...) expressions are not supported. They can be supported via a macro if so desired. (This author finds them pointless.)

• Erlang processes and message passing.

Some Scheme feature are not supported:

• No mutable aggregate data structures. This means no set-car!, string-set!, vector-set!, or similar procedures. This is due to inherent limitations in the Erlang/OTP VM.

• No call/cc. Supporting this would require a CPS-transform, which would make interoperability with Erlang code difficult. Note that the Erlang VM supports processes and exceptions, so call/cc is not needed to implement those features for ErlScheme.

• No "full numeric tower". This is mainly due to the Erlang/OTP VM only supporting integers (fixnums and bignums) and flonums.

• No variadic functions. The Erlang/OTP VM does not support this feature, and emulating it requires changing calling conventions which ends up making interoperability with Erlang code more difficult.

• No resumption from exception handlers. Like call/cc, supporting this would require a CPS-transform, making interoperability with Erlang code difficult.

ErlScheme is a Work In Progress

ErlScheme is far from finished, so here is an incomplete list of known omissions and planned extensions:

Omissions:

• Most of the Scheme standard bindings are not yet implemented.
• The R6RS/R7RS library system is not implemented, and may never be.
• No documentation.
• The macro / syntax system is old-fashioned and primitive.

Planned extensions:

• Write more of the system in ErlScheme itself.