README.md

purescript-subrecord

The SubRecord type, for encoding subrecords.

Installation

bower install purescript-subrecord

Overview

A Record x must contain values at all labels in the row x. For example, a value of type Record ( x :: Int, y :: String ) must contain an Int value at label x and a String value at label y.

This library introduces the type SubRecord x. A SubRecord x may contain values for the labels in row x. For example, a value of type SubRecord ( x :: Int, y :: String ) could be any of: {}, { x :: Int }, { y :: String} or { x :: String, y :: String }.

We can use mkSubRecord on a Record a, from which it creates a SubRecord r if a is a subrow of r.

testMkSubRecord :: SubRecord ( x :: Int, y :: String )
testMkSubRecord = mkSubRecord { x: 42 }

If we try to add a wrong label, the compiler will warn us.

wrongLabel :: SubRecord ( x :: Int, y :: String )
wrongLabel = mkSubRecord { z: 42 }
             ^^^^^^^^^^^^^^^^^^^^^
   could not match ( z :: Int | r ) with ( x :: Int, y :: String )

We can go back to a Record by providing default values for all labels in the row.

testWithDef :: { x :: Int, y :: String }
testWithDef = withDefaults { x: 0, y: "default" } testMkSubRecord

Then testWithDef.x evaluates to 42, set by testMkSubRecord, and testWithDef.y evaluates to "default", the given default for label y.