diff --git a/packages/argparse/src/argparse.test.ts b/packages/argparse/src/argparse.test.ts
index d71ae9b..06df487 100644
--- a/packages/argparse/src/argparse.test.ts
+++ b/packages/argparse/src/argparse.test.ts
@@ -1,9 +1,9 @@
-import {argparse} from './argparse'
+import {argparse, IArgsConfig} from './argparse'
 
 describe('argparse', () => {
 
   it('parses args', () => {
-    const args = argparse(['--one', '1', '--two', '2', '--four'], {
+    const args = argparse({
       one: {
         type: 'string',
         required: true,
@@ -14,8 +14,8 @@ describe('argparse', () => {
       },
       four: {
         type: 'boolean',
-      }
-    })
+      },
+    })(['--one', '1', '--two', '2', '--four'])
 
     const one: string = args.one
     const two: number = args.two
@@ -26,12 +26,120 @@ describe('argparse', () => {
     expect(four).toBe(true)
   })
 
+  describe('boolean', () => {
+    it('is not required, and is false by default', () => {
+      const result = argparse({
+        bool: {
+          type: 'boolean',
+        },
+      })([])
+      const value: boolean = result.bool
+      expect(value).toBe(false)
+    })
+    it('can be made required', () => {
+      expect(() => argparse({
+        bool: {
+          type: 'boolean',
+          required: true,
+        },
+      })([])).toThrowError(/Missing required args: bool/)
+    })
+    it('optionally accepts a true/false value', () => {
+      const parse = argparse({
+        bool: {
+          type: 'boolean',
+          alias: 'b',
+        },
+        other: {
+          type: 'string',
+        },
+      })
+      expect(parse(['--bool']).bool).toBe(true)
+      expect(parse(['--bool', 'false']).bool).toBe(false)
+      expect(parse(['--bool', 'true']).bool).toBe(true)
+      expect(parse(['--bool', '--other', 'value'])).toEqual({
+        bool: true,
+        other: 'value',
+      })
+    })
+    it('can be grouped by shorthand (single dash) notation', () => {
+      const parse = argparse({
+        a1: {
+          type: 'boolean',
+          alias: 'a',
+        },
+        b: {
+          type: 'boolean',
+          alias: 'c',
+        },
+        other: {
+          type: 'string',
+          alias: 'o',
+        },
+      })
+      expect(parse([])).toEqual({
+        a1: false,
+        b: false,
+        other: '',
+      })
+      expect(parse(['-ab'])).toEqual({
+        a1: true,
+        b: true,
+        other: '',
+      })
+      expect(parse(['-ca'])).toEqual({
+        a1: true,
+        b: true,
+        other: '',
+      })
+      expect(parse(['-abo', 'test'])).toEqual({
+        a1: true,
+        b: true,
+        other: 'test',
+      })
+      expect(() => parse(['-abo'])).toThrowError(/must be a string: -abo/)
+    })
+  })
+
+  describe('number', () => {
+    it('sets to NaN by default', () => {
+      const parse = argparse({
+        a: {
+          type: 'number',
+        },
+      })
+      expect(parse([])).toEqual({
+        a: NaN,
+      })
+      expect(() => parse(['-a'])).toThrowError(/must be a number: -a/)
+      expect(() => parse(['-a', 'no-number']))
+      .toThrowError(/must be a number: -a/)
+      expect(() => parse(['--a', 'no-number']))
+      .toThrowError(/must be a number: --a/)
+      expect(parse(['-a', '10'])).toEqual({
+        a: 10,
+      })
+      expect(parse(['--a', '11'])).toEqual({
+        a: 11,
+      })
+    })
+  })
+
   it('throws when required args missing', () => {
-    expect(() => argparse([], {
+    expect(() => argparse({
       one: {
         type: 'string',
         required: true,
       },
-    })).toThrowError(/missing required/i)
+    })([])).toThrowError(/missing required/i)
   })
+
+  it('throws when arg type is unknown', () => {
+    expect(() => argparse({
+      a: {
+        type: 'test',
+      } as any,
+    })(['-a'])).toThrowError(/Unknown type: test/)
+  })
+
 })
diff --git a/packages/argparse/src/argparse.ts b/packages/argparse/src/argparse.ts
index 6aa35c0..2011134 100644
--- a/packages/argparse/src/argparse.ts
+++ b/packages/argparse/src/argparse.ts
@@ -11,6 +11,7 @@ export type TArgType<T extends TArgTypeName> =
 export interface IArgConfig<T extends TArgTypeName> {
   type: T
   alias?: string
+  description?: string
   default?: TArgType<T>
   required?: boolean
 }
@@ -19,7 +20,7 @@ export interface IArgsConfig {
   [arg: string]: IArgConfig<TArgTypeName>
 }
 
-export type TArgs<T> = {
+export type TArgs<T extends IArgsConfig> = {
   [k in keyof T]: T[k] extends IArgConfig<infer A> ?
     TArgType<A> : never
 }
@@ -35,7 +36,7 @@ const iterate = <T>(arr: T[]) => {
     },
     peek(): T {
       return arr[i + 1]
-    }
+    },
   }
 }
 
@@ -45,56 +46,77 @@ function assert(cond: boolean, message: string) {
   }
 }
 
-export function argparse<T extends object>(
-  args: string[],
-  config: T extends IArgsConfig ? T : never,
-): TArgs<T> {
-  const result = {} as TArgs<T>
+function getDefaultValue(type: TArgTypeName) {
+  switch (type) {
+    case 'number':
+      return NaN
+    case 'string':
+      return ''
+    case 'boolean':
+      return false
+  }
+}
+
+export const argparse = <T extends IArgsConfig>(
+  config: T,
+) => (args: string[]): TArgs<T> => {
+  const result: any = {}
   const it = iterate(args)
 
-  const usedArgs: Record<string, true> = {}
   const aliases: Record<string, string> = {}
   const requiredArgs = Object.keys(config).reduce((obj, arg) => {
     const argConfig = config[arg]
-    if (argConfig.default !== undefined) {
-      result[arg] = argConfig.default
-    }
+    result[arg] = argConfig.default !== undefined
+      ? argConfig.default
+      : getDefaultValue(argConfig.type)
     if (argConfig.alias) {
+      assert(
+        argConfig.alias in aliases === false,
+        'Duplicate alias: ' + argConfig.alias)
       aliases[argConfig.alias] = arg
     }
-    obj[arg] = !!argConfig.required
+    if (argConfig.required) {
+      obj[arg] = true
+    }
     return obj
-  }, {} as Record<string, boolean>)
+  }, {} as Record<string, true>)
 
-  while(it.hasNext()) {
+  function getArgumentName(nameOrAlias: string): string {
+    return nameOrAlias in config ? nameOrAlias : aliases[nameOrAlias]
+  }
+
+  function processFlags(arg: string): string {
+    if (arg.substring(1, 2) === '-') {
+      return arg.substring(2)
+    }
+
+    const flags = arg.substring(1).split('')
+
+    flags.slice(0, flags.length - 1)
+    .forEach(flag => {
+      const argName = getArgumentName(flag)
+      const argConfig = config[argName]
+      assert(!!argConfig, 'Unknown argument: ' + flag)
+      assert(argConfig.type === 'boolean',
+        'The argument is not a flag/boolean: ' + flag)
+      delete requiredArgs[argName]
+      result[argName] = true
+    })
+
+    const lastArgName = getArgumentName(flags[flags.length - 1])
+    assert(!!lastArgName, 'Unknown argument: ' + lastArgName)
+    return lastArgName
+  }
+
+  while (it.hasNext()) {
     const arg = it.next()
     assert(arg.substring(0, 1) === '-', 'Arguments must start with -')
-    let argName: string
-    if (arg.substring(1, 2) !== '-') {
-      // flags
-      const flags = arg.substring(1).split('')
-
-      flags.slice(0, flags.length - 2)
-      .forEach(flag => {
-        const alias = aliases[flag]
-        const argConfig = config[alias]
-        assert(!!argConfig, 'Unknown flag: ' + flag)
-        assert(argConfig.type === 'boolean', 'The argument is not a flag/boolean: ' + flag)
-        delete requiredArgs[alias]
-        result[alias] = true
-      })
-
-      const lastArg = flags[flags.length - 1]
-      argName = aliases[lastArg]
-    } else {
-      argName = arg.substring(2)
-    }
+    const argName: string = processFlags(arg)
     const argConfig = config[argName]
     assert(!!argConfig, 'Unknown argument: ' + arg)
     delete requiredArgs[argName]
-    usedArgs[argName] = true
     const peek = it.peek()
-    switch(argConfig.type) {
+    switch (argConfig.type) {
       case 'string':
         assert(it.hasNext(), 'Value of argument must be a string: ' + arg)
         result[argName] = it.next()
@@ -116,12 +138,12 @@ export function argparse<T extends object>(
         }
         continue
       default:
-        throw new Error('Unknown type:' + argConfig.type)
+        assert(false, 'Unknown type: ' + argConfig.type)
     }
   }
 
   assert(!Object.keys(requiredArgs).length, 'Missing required args: ' +
-    Object.keys(requiredArgs).map(r => '--' + r))
+    Object.keys(requiredArgs).join(', '))
 
   return result
 }