Changes between Version 3 and Version 4 of Holes


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Timestamp:
Feb 10, 2012 11:04:57 AM (3 years ago)
Author:
xnyhps
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  • Holes

    v3 v4  
    11= Holes = 
    22 
    3 One of the features of the Emacs mode for [http://wiki.portal.chalmers.se/agda/pmwiki.php Agda] is the ability to add goals. By inserting a {{{?}}} in an expression, the compiler will introduce a hole. After loading the file (which typechecks it), Agda gives an overview of the holes in the file and their types. 
     3One of the features of the Emacs mode for [http://wiki.portal.chalmers.se/agda/pmwiki.php Agda] is the ability to add goals, as a placeholder for code that is yet to be written. By inserting a {{{?}}} in an expression, the compiler will introduce a hole. After loading the file (which typechecks it), Agda gives an overview of the holes in the file and their types. 
    44 
    55For example: 
     
    2424}}} 
    2525 
    26 As can be seen here, holes are numbered, and the typechecker returns the name for each of these holes. 
     26As can be seen here, holes are numbered, and the typechecker returns the inferred type for each of these holes. 
    2727 
    28 These goals can be useful as placeholders when writing code. They allow typechecking to continue although certain parts of code are missing and they make a good TODO list. 
     28These goals can make it a lot easier to write code. They allow typechecking to continue although certain parts of code are missing and they work as a TODO list. 
    2929 
    3030== In GHC == 
     
    3636As stated before, {{{undefined}}} typechecks just like a hole: it has type {{{a}}}, so it can be used anywhere. However, it is not very easy to use in this way: it is impossible to find out what type the compiler found for the hole, and it's impossible to get a list of all the holes used in your source file(s). 
    3737 
    38 The same example: 
     38A similar example: 
    3939{{{ 
    4040test :: [Bool] 
     
    4242}}} 
    4343 
    44 Will not help finding the types of the {{{undefined}}}s at all. One advantage is that the code will not refuse to run, unless one of the {{{undefined}}}s is actually evaluated. 
     44Will not help finding the types of the {{{undefined}}}s at all. One advantage is that the code can successfully run, except if one of the {{{undefined}}}s is actually evaluated. 
    4545 
    4646=== Implicit Parameters === 
    47 The GHC extension [http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#implicit-parameters Implicit Parameters] comes closer to how we'd expect holes to work. It makes it possible to specify a term with a question mark, denoting a implicit variable. 
     47The GHC extension [http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#implicit-parameters Implicit Parameters] comes closer to how holes are expected to work. This extension makes it possible to specify a term with a question mark, denoting a implicit variable, but this could also be seen as a hole. 
    4848 
    4949Same example: 
     
    7979}}} 
    8080 
    81 This will show you the type, however, it does consider it an error and fails, so there may be other problems you don't get to see because of it. It also will refuse to load and compile the module, so it's impossible to run the parts of it that are finished. 
     81This will show you the type, however, it is an error and aborts compilation, so there may be other problems you don't get to see because of it. It also will refuse to load and compile the module, so it's impossible to run the parts of it that are finished. 
    8282 
    8383The reason is that the hole becomes a part of the type signature, as a constraint. So to correctly use it here, the function would have to be written as: 
     
    8888}}} 
    8989 
    90 This makes it very impractical to use them as holes, as all type signatures have to be updated to let the typechecker continue. Not only in the functions that use the implicit parameter itself, but they propagate upwards, just like class constraints: if another function were to call {{{test}}}, it would have the same implicit parameters (and therefore, all of these type signatures would have to be updated when a new hole is added). Another tricky problem with implicit parameters is that implicit parameters with the same name in different functions are not assumed to be the same parameter (i.e., required to be unifiable), ''except'' if some function has both implicit parameters in its constraints. Lastly, it's impossible to run code with unbound implicit parameters, even if the parameters are never actually used. 
     90This makes it very impractical to use them as holes, as all type signatures have to be updated to let the typechecker continue. Not only in the functions that use the implicit parameter itself, but they propagate upwards, just like class constraints: if another function were to call {{{test}}}, it would have the same implicit parameters (and therefore, all of these type signatures would have to be updated when a new hole is added). 
     91 
     92Another thing to keep in mind with implicit parameters is that implicit parameters with the same name in different functions are not assumed to be the same (i.e., required to be unifiable), ''except'' if some function has both implicit parameters in its constraints. Lastly, it's impossible to run code with unbound implicit parameters, even if the parameters are never actually used. 
    9193 
    9294---- 
    9395 
    94 Now for possible ways we see holes working in GHC. 
     96Now for ways holes could be added to GHC(i). 
    9597 
    9698=== Agda-style === 
     
    112114 
    113115=== Named holes === 
    114 Implicit parameters have some good features too: they can be named, and so used in multiple locations. In the Agda-style, this would require let-binding a hole, which is a lot of effort for something that should be a temporary placeholder. So one idea is to allow giving holes a name, just like implicit parameters. 
     116Implicit parameters have some good features too: they can be named, and so used in multiple places. In the Agda-style, this would require let-binding a hole, which is a lot of effort for something that should be a temporary placeholder. So one idea is to allow giving holes a name, just like implicit parameters. 
    115117 
    116118For example: 
     
    132134}}} 
    133135 
    134 These could either be made shared within a module, or not (so not the confusing situation with implicit parameters, that are only shared when required). 
     136These could either be made shared within all functions in module, or not shared between functions at all (so not the confusing situation with implicit parameters, which are only shared when required). 
    135137 
    136138=== Not holes, ranges === 
    137 Holes can be useful for finding the type of something that still needs to be written, but a more general way of looking at it is this: it is currently quite easy to typecheck a complete expression, for example with {{{:t}}}, in GHCi. However, finding the type of a part of an expression within a module is hard. In a large, complicated function it could be useful to ask the compiler for the types subexpressions. {{{:t}}} does not help here, as the function's parameters and where/let/lambda-bound terms are not in scope. It would be useful if it were possible to annotate code to ask the compiler to give you the type found for it. 
     139Holes can be useful for finding the type of something that still needs to be written, but a more general way of looking at it is this: it is currently quite easy to typecheck a complete expression, for example with {{{:t}}}, in GHCi. However, finding the type of a part of an expression within a module is hard. In a large, complicated function it could be useful to ask the compiler for the types of certain subexpressions. {{{:t}}} does not help here, as the function's parameters and where/let/lambda-bound terms are not in scope. It would be useful if it were possible to annotate code to ask the compiler to give you the type found for the annotated expression. 
    138140 
    139141Simple example (let {{{{_ _}}}} denote a request for the type here):