6 ms·

Haskell is that rare beast: a language that Lispers may envy.

chriseidhof 17y ago
I, as a Haskeller, envy Lispers as well. Metaprogramming is so incredibly easy in Lisp. Oftentimes I can find good solutions in Haskell for things that people would normally do using metaprogramming, but still. And almost always the types really help me, but every now and then they get in the way.
- camccann 17y ago
  All I really want in a language is something similar to Haskell, with Lisp-like metaprogramming, easy support for Erlang-style concurrency, and syntax that doesn't immediately terrify the Blub programmers. Is that so much to ask?
  - fauigerzigerk 17y ago
    Yes I think that is too much to ask. Metaprogramming, I think, is by definition incompatible with compile time type checking. The "meta" in Metaprogramming means that the program is able to manipulate itself (recursively) at runtime. I believe it's theoretically impossible to combine that with AOT type checking and Haskell without its type system would not be Haskell at all. [edit] Of course you could argue that the sort of thing C++ does with templates is metaprogramming as well, only at compile time. In that case I suggest that we need another term for that because it's totally unlikle what Lisp is so good at.
    - ericb 17y ago
      The post above doesn't mention compile time type checking. Are you implying that to support one of the features he asks for that it is required?
      - camccann 17y ago
        One of Haskell's defining features is a very expressive and powerful static type system, so yeah, compile-time type inference/checking was pretty heavily implied by "similar to Haskell". And yes, the fact that I was essentially asking for a language with both static typing and runtime self-modification was the reason for the tongue-in-cheek "is that so much to ask". Might as well ask for a program that can compile any legal perl program [0]. That said, I suspect there's a lot that could be done to allow certain subsets of metaprogramming techniques in a static-typed language; some sort of crazy meta-type system that lets the compiler prove that something will only produce code with a particular polymorphic type, maybe? I don't know. [0] EDIT: My bad, it's actually parsing perl that's impossible, cf. http://www.perlmonks.org/?node_id=663393 http://www.perlmonks.org/?node_id=663393
        chriseidhof 17y ago
        Yes. A lot of the meta-programming is actually programming over the structure. For example, given a Java class, you could generate database code. In Java, this relies heavily on introspection, but you if you're interested in doing this in a type-safe way then you could look at Generic Programming (in Haskell). Also, you can generate code (at runtime) which is type-safe, compile the code, etc. Everything you ask for is possible in Haskell, however, it is definitely more complicated than in Lisp.
        fauigerzigerk 17y ago
        Compiling code at runtime may improve performance, but it doesn't give the kind of guarantees that static typing gives you. Static type checking proves that certain invariants hold at runtime and hence that certain defects are not possible. If a program can manipulate itself at runtime in arbitrary ways (not just reflect on itself in a read-only fashion), those proofs become invalid.
      - fauigerzigerk 17y ago
        Yes I do imply that. camccann puts it very well, and as I said, what would Haskell be without its static type system?
    - andrewcooke 17y ago
      you might want to google "staged programming" - it's what strongly typed languages do to get metaprogramming. (edit: afaik it doesn't allow the full "recursive manipulation" you describe, but it does let you generate code with code)
      - Hexstream 17y ago
        Thank you so much! I've had informal ideas quite like this from working on my "HTML rewriting system" part of my (unreleased) Common Lisp web framework. I now have a very specific search term to acquire more formalized knowledge which should speed up some future developments a lot because I won't have to independently come up with solutions to as many of the problems now.
        andrewcooke 17y ago
        heh. no problem. if you're interested in this kind of thing check out http://lambda-the-ultimate.com http://lambda-the-ultimate.com from time to time. just skimming the conversations can keep you up to date...
    - mainland 17y ago
      It's certainly possible for a typed language like ML or Haskell to support metaprogramming. As other have noted, Haskell has Template Haskell. However, systems like MetaML and MetaOCaml support metaprogramming and give much stronger typing guarantees than Template Haskell. See http://www.metaocaml.org/examples/ http://www.metaocaml.org/examples/ for inspiration.
      - fauigerzigerk 17y ago
        Sure, but there are theoretical limits to what runtime metprogramming can do whilst upholding the guarantees that a static type system provides. For instance, in order to check that a particular function call conforms to the function signature, the function signature must at least exist. Type checking a call to a function for which not even the signature exists anwhere within the system is impossible.
    - astine 17y ago
      A large part of what constitutes lisp-style metaprogramming is macros, which are definitely a compile-time feature.
      - fauigerzigerk 17y ago
        That's a very valid point. So I think we have these two types of metaprogramming, the runtime type and the compile time type. The latter is compatible with static typing but the former clearly is not.
        derefr 17y ago
        If the compiler is part of the runtime (i.e. an interpreter that compiles expressions in order to evaluate them), and then the type checker is part of the compiler, even eval should be able to throw type errors. What, then, is the problem?
        baguasquirrel 17y ago
        It's not a statically typed language like Haskell anymore?
        fauigerzigerk 17y ago
        The problem is that the type error occurs at runtime, which is exactly what static type checking is supposed to prevent in the first place.
        derefr 17y ago
        It depends on what you mean by "runtime." If you're creating a new datatype, is that really "at runtime," or are you just talking to the compiler interactively? To put it another way: metaprogramming might have effects at runtime, but not the kind of effects that change depending on what the runtime does. Metaprogramming should be deterministic for your program to be considered to be "in production."
    - messel 17y ago
      If I create syntax with a program (code with code) is that metaprogramming? Doesn't c++ or any text generating code already do this? Maybe it's not "native" and I have to run system level calls to force compiles and execution of created code. Almost all programs do metaprogramming (interpreted code, scala on the jvm, etc). What's the differentiatior for Lisps Metaprogramming (self contained languange structures?). I'm pretty interested in simplification of programmer interfaces on iteratively more functional (but perhaps more complex) underbellies. I will certainly investigate.
      - fauigerzigerk 17y ago
        My definition of metaprogramming is writing a program that manipulates itself. I'm not sure I would call any arbitrary code generator metaprogramming as this would cause the "self" in my definition suffer a severe identity crisis :-) But yes, C++ templates as well as Lisp macros are compile time metaprogramming. All languages that have eval and/or allow function/method bindings to be replaced at runtime allow runtime metaprogramming. Since static typing guarantees certain invariants it cannot be compatible with a program that violates those invariants at runtime. You could still decide to consider it metaprogramming when the program manipulates itself only within the limits of those guarantees, but when you look at what real world runtime metaprogramming is being used for (for instance in Rails) you will realise that these things (e.g method_missing) would not be possible within the limits of a statically typed language. [edit] Lisp makes both compile time and runtime metaprogramming exceptionally easy due to its homoiconic nature.
  - lsb 17y ago
    Lisp-like metaprogramming is really tricky, because a lot of the language that you interact with is defined by macros. The Haskell that you see is a thick layer of syntactic sugar over a much smaller Haskell core.
    - gruseom 17y ago
      Lisp-like metaprogramming is really tricky Whoa. That doesn't match my experience at all. I'd say it's simple and practically effortless (compared to what you'd have to do elsewhere). It would be surprising if it weren't, given that the whole language is organized around code=data.
      - scott_s 17y ago
        I think he meant Lisp-like metaprogramming in Haskell.
        gruseom 17y ago
        Ah. Thanks for the clarification.
  - yummyfajitas 17y ago
    Haskell has message passing concurrency within a single (OS) process. It's not quite erlang-style concurrency, but it gives you a lot of it. http://cvs.haskell.org/Hugs/pages/libraries/base/Control-Concurrent-Chan.html http://cvs.haskell.org/Hugs/pages/libraries/base/Control-Con... If someone built an AMQP library for haskell, we'd probably be 90% of the way there.
  - messel 17y ago
    All I want is a language which can tap directly in to my imagination and rapidly create machine functionality and simulations.
  - tumult 17y ago
    You should check out the Qi language, though the developer who has historically contributed the most to it has departed some months ago. It has the ideas you are looking for (sans message-passing concurrency, I think.)
  - pg 17y ago
    Metaprogramming and syntax don't go that well together. If you're going to manipulate programs effectively, you're going to be operating on source trees, and it's easiest to do that when source code and source tree are of the same form.
    - camccann 17y ago
      Thank you for picking up on that half of my joke. Everyone else went for the type system part... Not that it isn't easy to make a candied Lisp dialect with "friendly" syntax more appealing to non-Lisp programmers, but anyone who actually learns a Lisp quickly realizes that it just gets in the way... If I were to personally try creating an "ideal language" to match my tongue-in-cheek plea, I'd probably start with a terse Lisp-like syntax and a HM-like type system and go from there.
  - Ralith 17y ago
    Couldn't you obtain most of that within a Lisp library?
- samstokes 17y ago
  Does Template Haskell affect this discussion at all? (That's not a rhetorical question - I haven't tried any Template Haskell, so I'm genuinely interested to know what it's capable of and how it compares to Lisp macros.)
  - gwern 17y ago
    TH does a lot of the same stuff. On a practical level, it isn't as good, though. The implementation leaves a lot to be desired. For example, I once tried to use TH to check XMonad configurations at compile-times (specifically, that their keymaps were sensible), but turns out TH cannot operate on anything defined in the same module! See http://www.haskell.org/pipermail/xmonad/2008-November/006723.html http://www.haskell.org/pipermail/xmonad/2008-November/006723... (if you don't like context, scroll down to 'I had to admit failure').
- gradschool 17y ago
  Can you give an example of a situation where Haskell type checking gets in your way? Not to take issue, but because I'm a collector. I've been told that it's not that easy to make a list of functions in Haskell unless they all have the same input and output types, which seems restrictive to me.
  - jpr 17y ago
    Well, polymorphic lists are one thing that are somewhat awkward. Other thing is that a small change in some part of program (say introducing an additional parameter, or having to use IO or state) can require massive changes in rest of the program, or a PhD in category-theory.
  - tel 17y ago
    It's not "easy" to make a list, but instead maybe you would be able to work with a tuple (which is easy). More practically, Haskell is going to ask you implicitly why you want a heterogenous list of functions. In all likelihood they're going to have a common interface at some point and you abstract around that so that the list is still "homogenous" (which is harder, but still pretty easy).
albertcardona 17y ago
And a language that is more and more usable for non-pure math tasks.
jganetsk 17y ago
Isn't this problematic? Lispers fling around lots of superlatives, as if their favorite language will never be dethroned from its position atop the language kingdom. And they always felt justified about this. Shouldn't we confront them about this? I do think there's a science of programming languages which can inform us how to best design the tools we use. Isn't it just possible that these Hindley-Milner languages are the next step in the progression that LISP started? Look at the transition from Newtonian to Einsteinian mechanics. They were both great at the time of release, but one is clearly a step forward. The former is usually embeddable within the latter, but edge cases in the former don't quite work the same in the latter. And some cases in the former are just downright illegal in the latter. And unfortunately, the latter is a lot more complex for the power users. But, the latter is more expressive, teaches us more about the world... and enables completely new possibilities like nuclear fusion/fission. This is like the difference between dynamic and static typing. Dynamic typing is certainly embeddable within Haskell. Just define a data type Dyn which is the sum of types Int, String, Float, [Dyn], and Dyn -> Dyn. You will see how much LISP is possible without much added syntactic cruft. But Haskell users don't need and want to operate in this way, because they can leverage the benefits of static typing. Remember, these things don't come from whims of clever people or corporations. LISP and Haskell both come from research of very fundamental ideas. his is much more like real science than social science. And we should keep our eyes open to the future of this field, instead of proclaiming that we're finally done with our "100 year language".
- silentbicycle 17y ago
  There isn't some unified continuum of power in languages, though. There are some problems for which a H-M-ish type system is a tremendous advantage, and there are others for which it gets in the way. Same with lazy evaluation - it can make a problem much simpler or much harder to reason about. Sometimes using a language built from the ground up to support distributed and concurrent programming (e.g. Erlang), unification and backtracking (Prolog), or constraint programming is the right tool for the job. Sometimes being able to work close to the Unix kernel or run on an embedded system without an OS is more important. Languages are a means to an end -- they help manage complexity while solving a problem. There's value in being able to extend a language's semantics to support a superset of several major language families, but unless handled very carefully, it can turn the host language into a sprawling mess in the process.

I, as a Haskeller, envy Lispers as well. Metaprogramming is so incredibly easy in Lisp. Oftentimes I can find good solutions in Haskell for things that people would normally do using metaprogramming, but still. And almost always the types really help me, but every now and then they get in the way.

camccann 17y ago
All I really want in a language is something similar to Haskell, with Lisp-like metaprogramming, easy support for Erlang-style concurrency, and syntax that doesn't immediately terrify the Blub programmers. Is that so much to ask?
- fauigerzigerk 17y ago
  Yes I think that is too much to ask. Metaprogramming, I think, is by definition incompatible with compile time type checking. The "meta" in Metaprogramming means that the program is able to manipulate itself (recursively) at runtime. I believe it's theoretically impossible to combine that with AOT type checking and Haskell without its type system would not be Haskell at all. [edit] Of course you could argue that the sort of thing C++ does with templates is metaprogramming as well, only at compile time. In that case I suggest that we need another term for that because it's totally unlikle what Lisp is so good at.
  - ericb 17y ago
    The post above doesn't mention compile time type checking. Are you implying that to support one of the features he asks for that it is required?
    - camccann 17y ago
      One of Haskell's defining features is a very expressive and powerful static type system, so yeah, compile-time type inference/checking was pretty heavily implied by "similar to Haskell". And yes, the fact that I was essentially asking for a language with both static typing and runtime self-modification was the reason for the tongue-in-cheek "is that so much to ask". Might as well ask for a program that can compile any legal perl program [0]. That said, I suspect there's a lot that could be done to allow certain subsets of metaprogramming techniques in a static-typed language; some sort of crazy meta-type system that lets the compiler prove that something will only produce code with a particular polymorphic type, maybe? I don't know. [0] EDIT: My bad, it's actually parsing perl that's impossible, cf. http://www.perlmonks.org/?node_id=663393 http://www.perlmonks.org/?node_id=663393
      - chriseidhof 17y ago
        Yes. A lot of the meta-programming is actually programming over the structure. For example, given a Java class, you could generate database code. In Java, this relies heavily on introspection, but you if you're interested in doing this in a type-safe way then you could look at Generic Programming (in Haskell). Also, you can generate code (at runtime) which is type-safe, compile the code, etc. Everything you ask for is possible in Haskell, however, it is definitely more complicated than in Lisp.
        fauigerzigerk 17y ago
        Compiling code at runtime may improve performance, but it doesn't give the kind of guarantees that static typing gives you. Static type checking proves that certain invariants hold at runtime and hence that certain defects are not possible. If a program can manipulate itself at runtime in arbitrary ways (not just reflect on itself in a read-only fashion), those proofs become invalid.
    - fauigerzigerk 17y ago
      Yes I do imply that. camccann puts it very well, and as I said, what would Haskell be without its static type system?
  - andrewcooke 17y ago
    you might want to google "staged programming" - it's what strongly typed languages do to get metaprogramming. (edit: afaik it doesn't allow the full "recursive manipulation" you describe, but it does let you generate code with code)
    - Hexstream 17y ago
      Thank you so much! I've had informal ideas quite like this from working on my "HTML rewriting system" part of my (unreleased) Common Lisp web framework. I now have a very specific search term to acquire more formalized knowledge which should speed up some future developments a lot because I won't have to independently come up with solutions to as many of the problems now.
      - andrewcooke 17y ago
        heh. no problem. if you're interested in this kind of thing check out http://lambda-the-ultimate.com http://lambda-the-ultimate.com from time to time. just skimming the conversations can keep you up to date...
  - mainland 17y ago
    It's certainly possible for a typed language like ML or Haskell to support metaprogramming. As other have noted, Haskell has Template Haskell. However, systems like MetaML and MetaOCaml support metaprogramming and give much stronger typing guarantees than Template Haskell. See http://www.metaocaml.org/examples/ http://www.metaocaml.org/examples/ for inspiration.
    - fauigerzigerk 17y ago
      Sure, but there are theoretical limits to what runtime metprogramming can do whilst upholding the guarantees that a static type system provides. For instance, in order to check that a particular function call conforms to the function signature, the function signature must at least exist. Type checking a call to a function for which not even the signature exists anwhere within the system is impossible.
  - astine 17y ago
    A large part of what constitutes lisp-style metaprogramming is macros, which are definitely a compile-time feature.
    - fauigerzigerk 17y ago
      That's a very valid point. So I think we have these two types of metaprogramming, the runtime type and the compile time type. The latter is compatible with static typing but the former clearly is not.
      - derefr 17y ago
        If the compiler is part of the runtime (i.e. an interpreter that compiles expressions in order to evaluate them), and then the type checker is part of the compiler, even eval should be able to throw type errors. What, then, is the problem?
        baguasquirrel 17y ago
        It's not a statically typed language like Haskell anymore?
        fauigerzigerk 17y ago
        The problem is that the type error occurs at runtime, which is exactly what static type checking is supposed to prevent in the first place.
        derefr 17y ago
        It depends on what you mean by "runtime." If you're creating a new datatype, is that really "at runtime," or are you just talking to the compiler interactively? To put it another way: metaprogramming might have effects at runtime, but not the kind of effects that change depending on what the runtime does. Metaprogramming should be deterministic for your program to be considered to be "in production."
  - messel 17y ago
    If I create syntax with a program (code with code) is that metaprogramming? Doesn't c++ or any text generating code already do this? Maybe it's not "native" and I have to run system level calls to force compiles and execution of created code. Almost all programs do metaprogramming (interpreted code, scala on the jvm, etc). What's the differentiatior for Lisps Metaprogramming (self contained languange structures?). I'm pretty interested in simplification of programmer interfaces on iteratively more functional (but perhaps more complex) underbellies. I will certainly investigate.
    - fauigerzigerk 17y ago
      My definition of metaprogramming is writing a program that manipulates itself. I'm not sure I would call any arbitrary code generator metaprogramming as this would cause the "self" in my definition suffer a severe identity crisis :-) But yes, C++ templates as well as Lisp macros are compile time metaprogramming. All languages that have eval and/or allow function/method bindings to be replaced at runtime allow runtime metaprogramming. Since static typing guarantees certain invariants it cannot be compatible with a program that violates those invariants at runtime. You could still decide to consider it metaprogramming when the program manipulates itself only within the limits of those guarantees, but when you look at what real world runtime metaprogramming is being used for (for instance in Rails) you will realise that these things (e.g method_missing) would not be possible within the limits of a statically typed language. [edit] Lisp makes both compile time and runtime metaprogramming exceptionally easy due to its homoiconic nature.
- lsb 17y ago
  Lisp-like metaprogramming is really tricky, because a lot of the language that you interact with is defined by macros. The Haskell that you see is a thick layer of syntactic sugar over a much smaller Haskell core.
  - gruseom 17y ago
    Lisp-like metaprogramming is really tricky Whoa. That doesn't match my experience at all. I'd say it's simple and practically effortless (compared to what you'd have to do elsewhere). It would be surprising if it weren't, given that the whole language is organized around code=data.
    - scott_s 17y ago
      I think he meant Lisp-like metaprogramming in Haskell.
      - gruseom 17y ago
        Ah. Thanks for the clarification.
- yummyfajitas 17y ago
  Haskell has message passing concurrency within a single (OS) process. It's not quite erlang-style concurrency, but it gives you a lot of it. http://cvs.haskell.org/Hugs/pages/libraries/base/Control-Concurrent-Chan.html http://cvs.haskell.org/Hugs/pages/libraries/base/Control-Con... If someone built an AMQP library for haskell, we'd probably be 90% of the way there.
- messel 17y ago
  All I want is a language which can tap directly in to my imagination and rapidly create machine functionality and simulations.
- tumult 17y ago
  You should check out the Qi language, though the developer who has historically contributed the most to it has departed some months ago. It has the ideas you are looking for (sans message-passing concurrency, I think.)
- pg 17y ago
  Metaprogramming and syntax don't go that well together. If you're going to manipulate programs effectively, you're going to be operating on source trees, and it's easiest to do that when source code and source tree are of the same form.
  - camccann 17y ago
    Thank you for picking up on that half of my joke. Everyone else went for the type system part... Not that it isn't easy to make a candied Lisp dialect with "friendly" syntax more appealing to non-Lisp programmers, but anyone who actually learns a Lisp quickly realizes that it just gets in the way... If I were to personally try creating an "ideal language" to match my tongue-in-cheek plea, I'd probably start with a terse Lisp-like syntax and a HM-like type system and go from there.
- Ralith 17y ago
  Couldn't you obtain most of that within a Lisp library?
samstokes 17y ago
Does Template Haskell affect this discussion at all? (That's not a rhetorical question - I haven't tried any Template Haskell, so I'm genuinely interested to know what it's capable of and how it compares to Lisp macros.)
- gwern 17y ago
  TH does a lot of the same stuff. On a practical level, it isn't as good, though. The implementation leaves a lot to be desired. For example, I once tried to use TH to check XMonad configurations at compile-times (specifically, that their keymaps were sensible), but turns out TH cannot operate on anything defined in the same module! See http://www.haskell.org/pipermail/xmonad/2008-November/006723.html http://www.haskell.org/pipermail/xmonad/2008-November/006723... (if you don't like context, scroll down to 'I had to admit failure').
gradschool 17y ago
Can you give an example of a situation where Haskell type checking gets in your way? Not to take issue, but because I'm a collector. I've been told that it's not that easy to make a list of functions in Haskell unless they all have the same input and output types, which seems restrictive to me.
- jpr 17y ago
  Well, polymorphic lists are one thing that are somewhat awkward. Other thing is that a small change in some part of program (say introducing an additional parameter, or having to use IO or state) can require massive changes in rest of the program, or a PhD in category-theory.
- tel 17y ago
  It's not "easy" to make a list, but instead maybe you would be able to work with a tuple (which is easy). More practically, Haskell is going to ask you implicitly why you want a heterogenous list of functions. In all likelihood they're going to have a common interface at some point and you abstract around that so that the list is still "homogenous" (which is harder, but still pretty easy).

albertcardona 17y ago

And a language that is more and more usable for non-pure math tasks.

jganetsk 17y ago

Isn't this problematic? Lispers fling around lots of superlatives, as if their favorite language will never be dethroned from its position atop the language kingdom. And they always felt justified about this. Shouldn't we confront them about this? I do think there's a science of programming languages which can inform us how to best design the tools we use. Isn't it just possible that these Hindley-Milner languages are the next step in the progression that LISP started? Look at the transition from Newtonian to Einsteinian mechanics. They were both great at the time of release, but one is clearly a step forward. The former is usually embeddable within the latter, but edge cases in the former don't quite work the same in the latter. And some cases in the former are just downright illegal in the latter. And unfortunately, the latter is a lot more complex for the power users. But, the latter is more expressive, teaches us more about the world... and enables completely new possibilities like nuclear fusion/fission. This is like the difference between dynamic and static typing. Dynamic typing is certainly embeddable within Haskell. Just define a data type Dyn which is the sum of types Int, String, Float, [Dyn], and Dyn -> Dyn. You will see how much LISP is possible without much added syntactic cruft. But Haskell users don't need and want to operate in this way, because they can leverage the benefits of static typing. Remember, these things don't come from whims of clever people or corporations. LISP and Haskell both come from research of very fundamental ideas. his is much more like real science than social science. And we should keep our eyes open to the future of this field, instead of proclaiming that we're finally done with our "100 year language".

silentbicycle 17y ago
There isn't some unified continuum of power in languages, though. There are some problems for which a H-M-ish type system is a tremendous advantage, and there are others for which it gets in the way. Same with lazy evaluation - it can make a problem much simpler or much harder to reason about. Sometimes using a language built from the ground up to support distributed and concurrent programming (e.g. Erlang), unification and backtracking (Prolog), or constraint programming is the right tool for the job. Sometimes being able to work close to the Unix kernel or run on an embedded system without an OS is more important. Languages are a means to an end -- they help manage complexity while solving a problem. There's value in being able to extend a language's semantics to support a superset of several major language families, but unless handled very carefully, it can turn the host language into a sprawling mess in the process.