expr

Simple parallel expression engine for erlang.

Usage

Forms and expressions

Every form and/or has a type field. It also should set line to get helpful errors that map to the correct line of the original source. value and children can be present depending on the type.

calls, assignments and variables may also have the following fields:

silent - return undefined if the expression fails
timeout - timeout after ms
spawn - spawn the function in a wrapper process for an async response

Types

Literals

atom

#{
  type => literal,
  value => atom
}

binary

#{
  type => literal,
  value => <<"binary">>
}

float

#{
  type => literal,
  value => 3.14
}

integer

#{
  type => literal,
  value => 123
}

Compound

list

#{
  type => list,
  children => [
    #{
      type => literal,
      value => first
    },
    #{
      type => literal,
      value => second
    },
    #{
      type => literal,
      value => third
    }
  ]
}

tuple

#{
  type => tuple,
  children => [
    #{
      type => literal,
      value => first
    },
    #{
      type => literal,
      value => second
    },
    #{
      type => literal,
      value => third
    }
  ]
}

map

#{
  type => map,
  children => [
    #{
      type => tuple,
      children => [
        #{
          type => literal,
          value => first_key
        },
        #{
          type => literal,
          value => first_value
        }
      ]
    },
    #{
      type => tuple,
      children => [
        #{
          type => literal,
          value => second_key
        },
        #{
          type => literal,
          value => second_value
        }
      ]
    },
    #{
      type => tuple,
      children => [
        #{
          type => literal,
          value => third_key
        },
        #{
          type => literal,
          value => third_value
        }
      ]
    }
  ]
}

functions

call

#{
  type => call,
  value => {module, function},
  children => [
    #{
      type => literal,
      value => first_argument
    },
    #{
      type => literal,
      value => second_argument
    }
    #{
      type => literal,
      value => third_argument
    }
  ]
}

cond

#{
  type => 'cond',
  children => [
    %% should return 'true' or 'false'
    #{
      type => call,
      value => {module, is_valid}
    },
    %% only called if 'true'
    #{
      type => call,
      value => {module, truths}
    },
    %% only called if 'false'
    #{
      type => call,
      value => {module, falsities}
    }
  ]
}

#{
  type => 'cond',
  children => [
    %% should return 'true' or 'false'
    #{
      type => call,
      value => {module, is_valid}
    },
    %% only called if 'true'
    #{
      type => call,
      value => {module, truths}
    }
    %% otherwise returns 'undefined'
  ]
}

comprehension

#{
  type => comprehension,
  children => [
    #{
      type => assign,
      value => 'User',
      children => [
        #{
          type => list,
          children => [
            #{
              type => literal,
              value => <<"1">>
            },
            #{
              type => literal,
              value => <<"2">>
            },
            #{
              type => literal,
              value => <<"3">>
            },
            #{
              type => literal,
              value => <<"4">>
            },
            #{
              type => literal,
              value => <<"5">>
            }
          ]
        }
      ]
    },
    #{
      type => call,
      value => {maps, get},
      children => [
        #{
          type => literal,
          value => name
        },
        #{
          type => variable,
          value => 'User'
        }
      ]
    }
  ]
}.

Variables

assign

#{
  type => assign,
  value => 'MyVariable',
  children => [
    #{
      type => literal,
      value => <<"my value">>
    }
  ]
}

variable

#{
  type => variable,
  value => 'MyVariable'
}

Runtime

The runtime behaves lazily, meaning it only evaluates the expressions when it needs the answer. It also uses a kind of error monad to handle exceptions.

loop

each expr in pending
- is it root and is it done?
  - yes
    - return the value from values
  - no
    - are the dependencies ready?
      - yes
        
        push the expr on fns
      - no
        
        push the dependencies on pending
while messages
- recieve with timeout 0
  - timeout
    - exit loop
  - value
    - set the return value in values from pid id
    - remove pid from pids
  - error
    - push error on errors
each fn in fns
- execute function with arguments
  - error
    - push error on errors
  - pid
    - push pid on pids with a start time
  - success
    - set the return value in values from fn id

when an error occurs

each error in errors
- is silent?
  - set fn result to undefined
- otherwise
  - crash

Tests

$ make test

camshaft/expr